I write mostly frontends. Sometimes the APIs I talk to give back beautiful English error messages - that I can't just show to the user, because they are using a different language most of the time. And I don't want to write logic that depends on that sentence, far too brittle.
Right—I think the “error code” here is going to be the error type, i.e., UsernameError, or some qualified version of that.
It’s not perfect, but software evolves through many imperfect stages as it gets better, and this is one such imperfect stage that your software may evolve through.
Including a human-readable version of the error is useful because the developers / operators will want to read through the logs for it. Sometimes that is where you stop, because not all errors from all backends will need to be localized.
