Return error codes from API

[denolfe]

As of now, any handling of errors on the frontend has to be done via HTTP status code or error message. The introduction of i18n makes using error messages much more difficult. Having a set list of error codes maintained internally would make handling these much easier.

Ideally, in addition to the error message, we'd send back a field like errorCode which would be standardized.

[dolanoadriano]

ha, how's it going? :D