Add endpoints.onErrorAdd

[PhilippKolmann]

Summary

Add endpoints[].onErrorAdd to add response body or parts of it as error, when the conditions fail for easier debugging.

Implements #742

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[TwiN]

I would be more okay with having the debug parameter print the body in the logs honestly.
The big problem is that it's pretty common for firewalls or outages to cause normally JSON or plain text endpoints to return HTML pages, and those HTML pages can be pretty lengthy, something that may cause issues both at the storage level and at the display level on the UI

[PhilippKolmann]

My intention was, that I see immediately what my micro services are lacking because they write it in the body.

But I see your point.

[PhilippKolmann]

Would you prefer to have a limit of text that could be added, and if the body is over the limit, then to truncate or not print it?