Adapting errorhandling to rfc7807

[ue71603]

We had it mentionend, that there's an internet standard to model to carry machine-readable details of errors:

https://datatracker.ietf.org/doc/html/rfc7807
The idea would be to rely heavier on this standards and to change the naming: The proposal is to use "Problem" instead of "ErrorMessage". This also facilitates the implementation of OJP in REST.

from: VDVde/OJP#167