Fix Internal Server Error when OIDC token request fails for whatever reason
Due to a programming oversight, we currently throw away the HttpResponse and return None when an OIDC token request fails in the backend. This results in a 500 Internal Server Error.
This MR fixes it so we correctly return a response, so at least the client gets some insight into the error.
I also added the error_description field to the client response, so debugging Keycloak issues become easier.