Improve error handling

[josvandervelde]

The default error handling of FastAPI is a bit weird. You get response bodies such as

{"detail": 
    [
        {"loc": ["body", "title"], "msg": "field required", "type": "value_error.missing"}
    ]
}

This is not conform https://datatracker.ietf.org/doc/html/rfc7807. We want to conform to an international standard. As far as I know, rfc7807 is the most used international standard for error handling.

TODOs:

1. check if rfc7807 is the best international standard to comply to
2. make sure errors are rfc7807 compliant
3. maybe check all the errors that are thrown around, and improve on then, to improve the user /developers experience

[Taniya-Das]

1. As it seems RFC 7807 is still the most widely recognized international standard. The standard structure seems suitable to our work.