Nice Golang Protobuf JSONPB

The jsonpb implementation of golang/protobuf has very bad error handling, making it hard to return human-understandable errors.

This is a fork of the Unmarshal functionality of jsonpb with fixes for error handling:

• Errors are are now prefixed with a "stack" path of fields that are returned in
• Poor "cannot deserialize into `json.RawMessage" errors now use proper types
• Unknown fields now return a helpful message listing known fields :)

Relevant upstream Issue: https://github.com/golang/protobuf/issues/266