Add client/server indication to OAuth2 error messages

[reinkrul]

When the EHR calls the local Nuts node to interact with a remote OAuth2 server, it's unclear whether error messages come from the local Nuts node, or the remote one. E.g.:

{
  "detail": "invalid_request - unable to get subject DID from VC: credential subjects have no ID",
  "status": 400,
  "title": "RequestServiceAccessToken failed"
}

This error came from the remote system, but the same checks are performed (thus, similar messages could be returned from) local Nuts node.

We need a very clear way to indicate who generated the error message, so developers/administrators know where to fix the issue.