Make Katello api error rendering consistent with Foreman's
Right now a lot the error messages are rendered differently across Foreman and Katello's APIs
Also, the use of the attribute displayMessage should be limited to the candlepin_proxies_controller where we need to return messages to RHSM in the specific format that it requires.
Katello: https://github.com/Katello/katello/blob/master/app/lib/katello/api/v2/error_handling.rb (respond_for_exception method).
So that the api errors are presented consistently across Foreman/Katello, It would be ideal for Katello to share rendering code.