Documentation of API error codes

[s-hamann]

The docs currently do not contain a lot of information on possible error codes and conditions returned by the API. API clients therefore need to implement error handling based on observations and assumptions, and can not rely on documented behaviour.

It would be nice to have a reference of possible error conditions and HTTP status codes for each endpoint.
Furthermore, I noticed that errors seem to contain JSON data with a human-readable error message in the detail field. Is this always the case? At a quick glance, it seems to be. But I could not find any reliable information.

I believe, #359 has the potential to naturally provide this kind of information. In the mean time, a human-readable documentation would help.