Improve handling of httperrors.Error

[morremeyer]

The backend should handle errors as the error type if possible. With the current approach, a lot of code is duplicated.

See https://go.dev/doc/faq#nil_error for details.
However, we also want to transport the HTTP status code. This needs some more thinking and research.

It might be possible to have a generic handler function for the errors that takes the Response type as parameter via a suitable interface and can then return the correct response. With that, the handling code could possibly be much shorter.

[morremeyer]

We can't use the error type in response structs since an error marshals to {}, see golang/go#5161.

Possible solution: https://go.dev/play/p/NrtoYsGzij