feat: wrap external errors instead of changing them to root errors

[morningvera]

this PR is a redesign of the way eris wraps external errors. instead of unwrapping them and disguising them as root errors, it simply wraps them to maintain all error context. closes #82 and closes #81.

[morningvera]

I added a test using the Golang As and confirmed that it works now. We can add our own version of it in another PR.

[morningvera]

Reminder to update the readme to reflect external error handling changes

[sum2000]

we may need to modify/add examples

[sum2000]

Since we are adding external error in root error, the New() method should be capable of the same. I should be able to create a new root error from external error. what do you think?

[sum2000]

Reviewed the code, reviewing the tests locally

[morningvera]

Since we are adding external error in root error, the New() method should be capable of the same. I should be able to create a new root error from external error. what do you think?

wouldn't that be equivalent to wrapping an external error? i think the method definition would look something like NewWithExt(ext error, msg string), which is the same definition and implementation of our wrap methods so this use case is already covered

[sum2000]

Since we are adding external error in root error, the New() method should be capable of the same. I should be able to create a new root error from external error. what do you think?

wouldn't that be equivalent to wrapping an external error? i think the method definition would look something like NewWithExt(ext error, msg string), which is the same definition and implementation of our wrap methods so this use case is already covered

It won't be equivalent to wrapping because the wrap method returns wrap error type and the proposed method would return root error type

[morningvera]

that's actually incorrect. for the external error case, wrap returns a root error.

https://github.com/rotisserie/eris/pull/84/files#diff-0bccded79c042f8676aa4c5a07863306R78

[codecov-io]

Codecov Report

Merging #84 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
- Coverage   98.91%   98.90%   -0.02%     
==========================================
  Files           3        3              
  Lines         277      274       -3     
==========================================
- Hits          274      271       -3     
  Misses          1        1              
  Partials        2        2              

Impacted Files	Coverage Δ
eris.go	100.00% <100.00%> (ø)
format.go	100.00% <100.00%> (ø)

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