We currently test for malformed responses and 404s but not for 200 responses that contain an API error message, such as:

{
  "error": {
    "code": "unknown_action",
    "info": "Unrecognized value for parameter \"action\": oscillate.",
    "*": "See https://en.wikipedia.org/w/api.php for API usage."
  },
  "servedby": "mw1286"
}

We should add such tests to our Retrofit MW API clients to ensure that they're handling these error responses correctly.