Error objects in error responses include lots of None elements

[jfinkels]

Error responses currently look like this:

{
  "errors": [
    {
      "code": null, 
      "detail": "Accept header, if specified, must be the JSON API media type: application/vnd.api+json", 
      "id": null, 
      "links": null, 
      "meta": null, 
      "source": null, 
      "status": 406, 
      "title": null
    }
  ], 
  "jsonapi": {
    "version": "1.0"
  }, 
  "meta": {}
}

These shouldn't really be nulls, they should be filled in with the appropriate data, or at least removed.