Refactor error_source.go

[adamyeats]

This PR includes a number of changes to error_source.go:

• SourceError renamed to ErrorWithSource to be more explicit about intention. Arguments have been flipped so that err is passed in the first position to be more consistent with the other functions in the error_source package.
• ErrorWithSource uses the functional options pattern to make it more extensible via ErrorSourceOptions, and make the bool passed as the last param optional (defaults to false). An override can be set by using calling WithOverride, e.g. ErrorWithSource(err, source, WithOverride())
• Response renamed to ResponseWithErrorSource to be more explicit about intention