_assertStatusArray(statusArray, res) {
    if (!statusArray.includes(res.status)) {
      const b = STATUS_CODES[res.status];
      const expectedList = statusArray.join(', ');
      return new Error(
        'expected one of "' + expectedList + '", got ' + res.status + ' "' + b + '"'
      );
    }
  }
Perform assertions on the response status and return an Error upon failure.

@param {Array<Number>} statusArray
@param {Response} res
@return {?Error}
@api private