Fix a glitch on raising exception when source doesn't exist.

[mozamimy]

I found a glitch when source file or directory doesn't exist.

For example, there is a itamae recipe file,

remote_directory "/path/to/destination" do
  source "/path/to/source"
end

When /path/to/source does not exist, itamae says following exception message,

backend.rb:130:in `send_directory': uninitialized constant Itamae::Backend::Base::Error (NameError)

So I fixed the problem by using SourceNotExistError.

[ryotarai]

Thank you!

[ryotarai]

released v1.2.15