nginx エラーページでjsonを返す方法

  error_page 400 404 405 =200 @40*_json;

  location @40*_json {
    default_type application/json;
    return 200 '{"code":"1", "message": "Not Found"}';
  }

  error_page 500 502 503 504 =200 @50*_json;

  location @50*_json {
    default_type application/json;
    return 200 '{"code":"1", "message": "Unknown Error"}';
  }