fix(error): Improve access denied error messaging

[jonsie]

This is the first part of the change to improve front50's access denied error messaging. After this change we will replace front50's AccessDeniedExceptionHandler with FiatAccessDeniedExceptionHandler.

Here are examples of the different error message contexts:

//Non service account example:
{
  "timestamp": "2019-09-25T18:41:21.833+0000",
  "status": 403,
  "error": "Forbidden",
  "message": "Access denied to application smalleytest - required authorization: WRITE"
}

//Service account example:
{
  "timestamp": "2019-09-25T18:44:27.137+0000",
  "status": 403,
  "error": "Forbidden",
  "message": "Access denied to serviceaccount someServiceAccount"
}