Fixes #637; Preserve Err values when using Json.Decode.customDecoder

[noahzgordon]

See issue: https://github.com/elm-lang/core/issues/637

This commit allows Err values returned by the second argument of Json.Decode.customDecoder to be preserved when the decoder fails. This way all user-defined error messages are not swallowed by the decoder.

[evancz]

I just ran into this independently and did a fix in 7f6e518. Someone pointed me here, which is very similar. My fix includes context information, so if you run into a customDecode nine levels into a JSON object, the error message will tell you about that context.

That said, it's probably still a good idea to test that the error message produced contains the Err str string as a subset. Are you willing to adapt your tests to the alternate implementation?

[evancz]

I just ran into this independently and did a fix in 7f6e518. Someone pointed me here, which is very similar. My fix includes context information, so if you run into a customDecode nine levels into a JSON object, the error message will tell you about that context.

That said, it's probably still a good idea to test that the error message produced contains the Err str string as a subset. Are you willing to adapt your tests to the alternate implementation?

[noahzgordon]

@evancz Gladly! I will open a new PR shortly.

[noahzgordon]

@evancz Gladly! I will open a new PR shortly.