message: Promote additionalProperties to a dictionary
diff: "message": {
  "type": "string",
  "description": "A human readable error message."
  + }
  },
  "additionalProperties": {
  - "type": "object",
  - "description": "Additional custom properties."
  - }
  + "type": "object"
  },
  "required": [
  "code",
  