Serve REST errors as application/api-problem+json OR application/vnd.error+json

Problem/Motivation

As was discussed in #1865594: Better REST error messages in case of exceptions, we want to respond with clearer error messages. There seem to be two different approaches which are gaining traction, api-problem from the IETF and vnd.error from the HAL community.

Proposed resolution

Decide which to use by default and implement it in a way that makes it easy to change.