JSON unmarshaler returns UnknownFieldError error

[atombender]

This makes the JSON unmarshaler return a new error struct, UnknownFieldError, for unknown fields, when AllowUnknownFields is false. This is necessary in order for callers to be able to hide Go internals in APIs. Previously, the error of Unmarshal would be an error string such as:

unknown field "glorp" in proto.Mutation

This leaks the type proto.Mutation to clients, which should not need to see such internals when they are consuming a pure-JSON HTTP API.

There are other instances of fmt.Errorf being used, but this is the main one that leaks.

[atombender]

Hold on, I'll add a test for this struct.

[dsnet]

Before adding API to distinguish between different json errors, there should first be an issue that discusses what the use-case for this even should be. After that, we can figure out what the right API is. A new error type is not necessarily the API we would want to go with.

Feel free to open an issue justifying the ability to distinguish an unknown field error.

[atombender]

I created an issue. Hopefully it won't take another year for this to be resolved.