Catch Pkg errors and display a nicer error instead

[SaschaMann]

When a student tries to load a package through Pkg.add, the error is rather cryptic. We could probably catch that and display a nicer error instead.

Or overload Pkg methods with a simple error message like Pkg operations are not supported by the test runner.

LoadError: LoadError: failed to clone from https://github.com/JuliaArrays/OffsetArrays.jl, error: GitError(Code:ERROR, Class:Net, failed to resolve address for github.com: Temporary failure in name resolution)
Stacktrace:
  [1] pkgerror(msg::String)
    @ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:55
  [2] clone(ctx::Pkg.Types.Context, url::String, source_path::String; header::Nothing, credentials::Nothing, kwargs::Base.Iterators.Pairs{Symbol, Bool, Tuple{Symbol}, NamedTuple{(:isbare,), Tuple{Bool}}})
    @ Pkg.GitTools [...]/julia/stdlib/v1.6/Pkg/src/GitTools.jl:119
  [3] #ensure_clone#2
    @ [...]/julia/stdlib/v1.6/Pkg/src/GitTools.jl:72 [inlined]
  [4] handle_repo_add!(ctx::Pkg.Types.Context, pkg::Pkg.Types.PackageSpec)
    @ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:619
  [5] handle_repos_add!(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec})
    @ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:686
  [6] add(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; preserve::Pkg.Types.PreserveLevel, platform::Base.BinaryPlatforms.Platform, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:177
  [7] add
    @ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:148 [inlined]
  [8] add(pkgs::Vector{Pkg.Types.PackageSpec}; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:73
  [9] add
    @ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:72 [inlined]
 [10] #add#20
    @ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:69 [inlined]
 [11] add
    @ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:69 [inlined]
 [12] add(; name::Nothing, uuid::Nothing, version::Nothing, url::String, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:91
 [13] top-level scope
    @ ./pangram.jl:2
 [14] include(mod::Module, _path::String)
    @ Base ./Base.jl:386
 [15] include(x::String)
    @ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:1
 [16] top-level scope
    @ ./runtests.jl:3
 [17] include(mod::Module, _path::String)
    @ Base ./Base.jl:386
 [18] include
    @ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:1 [inlined]
 [19] macro expansion
    @ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:14 [inlined]
 [20] macro expansion
    @ [...]/julia/stdlib/v1.6/Test/src/Test.jl:1151 [inlined]
 [21] macro expansion
    @ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:14 [inlined]
 [22] macro expansion
    @ ~/.julia/packages/Suppressor/nTjgZ/src/Suppressor.jl:127 [inlined]
 [23] runtests(testfile::String)
    @ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:13
 [24] test_runner(exercise_slug::String, solution_dir::String)
    @ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:18
 [25] test_runner(exercise_slug::String, solution_dir::String, output_dir::String)
    @ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:39
 [26] top-level scope
    @ /opt/test-runner/run.jl:3
 [27] include(mod::Module, _path::String)
    @ Base ./Base.jl:386
 [28] exec_options(opts::Base.JLOptions)
    @ Base ./client.jl:285
 [29] _start()
    @ Base ./client.jl:485
in expression starting at ./pangram.jl:2
in expression starting at ./runtests.jl:3

caused by: GitError(Code:ERROR, Class:Net, failed to resolve address for github.com: Temporary failure in name resolution)
Stacktrace:
  [1] macro expansion
    @ [...]/julia/stdlib/v1.6/LibGit2/src/error.jl:110 [inlined]
  [2] clone(repo_url::SubString{String}, repo_path::String, clone_opts::LibGit2.CloneOptions)
    @ LibGit2 [...]/julia/stdlib/v1.6/LibGit2/src/repository.jl:459
  [3] clone(repo_url::SubString{String}, repo_path::String; branch::String, isbare::Bool, remote_cb::Ptr{Nothing}, credentials::LibGit2.CachedCredentials, callbacks::Dict{Symbol, Tuple{Ptr{Nothing}, Any}})
    @ LibGit2 [...]/julia/stdlib/v1.6/LibGit2/src/LibGit2.jl:583
  [4] clone(ctx::Pkg.Types.Context, url::String, source_path::String; header::Nothing, credentials::Nothing, kwargs::Base.Iterators.Pairs{Symbol, Bool, Tuple{Symbol}, NamedTuple{(:isbare,), Tuple{Bool}}})
    @ Pkg.GitTools [...]/julia/stdlib/v1.6/Pkg/src/GitTools.jl:109
  [5] #ensure_clone#2
    @ [...]/julia/stdlib/v1.6/Pkg/src/GitTools.jl:72 [inlined]
  [6] handle_repo_add!(ctx::Pkg.Types.Context, pkg::Pkg.Types.PackageSpec)
    @ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:619
  [7] handle_repos_add!(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec})
    @ Pkg.Types [...]/julia/stdlib/v1.6/Pkg/src/Types.jl:686
  [8] add(ctx::Pkg.Types.Context, pkgs::Vector{Pkg.Types.PackageSpec}; preserve::Pkg.Types.PreserveLevel, platform::Base.BinaryPlatforms.Platform, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:177
  [9] add
    @ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:148 [inlined]
 [10] add(pkgs::Vector{Pkg.Types.PackageSpec}; kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:73
 [11] add
    @ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:72 [inlined]
 [12] #add#20
    @ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:69 [inlined]
 [13] add
    @ [...]/julia/stdlib/v1.6/Pkg/src/API.jl:69 [inlined]
 [14] add(; name::Nothing, uuid::Nothing, version::Nothing, url::String, rev::Nothing, path::Nothing, mode::Pkg.Types.PackageMode, subdir::Nothing, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ Pkg.API [...]/julia/stdlib/v1.6/Pkg/src/API.jl:91
 [15] top-level scope
    @ ./pangram.jl:2
 [16] include(mod::Module, _path::String)
    @ Base ./Base.jl:386
 [17] include(x::String)
    @ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:1
 [18] top-level scope
    @ ./runtests.jl:3
 [19] include(mod::Module, _path::String)
    @ Base ./Base.jl:386
 [20] include
    @ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:1 [inlined]
 [21] macro expansion
    @ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:14 [inlined]
 [22] macro expansion
    @ [...]/julia/stdlib/v1.6/Test/src/Test.jl:1151 [inlined]
 [23] macro expansion
    @ ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:14 [inlined]
 [24] macro expansion
    @ ~/.julia/packages/Suppressor/nTjgZ/src/Suppressor.jl:127 [inlined]
 [25] runtests(testfile::String)
    @ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/runner.jl:13
 [26] test_runner(exercise_slug::String, solution_dir::String)
    @ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:18
 [27] test_runner(exercise_slug::String, solution_dir::String, output_dir::String)
    @ ExercismTestReports ~/.julia/packages/ExercismTestReports/Ov0Sg/src/ExercismTestReports.jl:39
 [28] top-level scope
    @ /opt/test-runner/run.jl:3
 [29] include(mod::Module, _path::String)
    @ Base ./Base.jl:386
 [30] exec_options(opts::Base.JLOptions)
    @ Base ./client.jl:285
 [31] _start()
    @ Base ./client.jl:485