_assertStatus(status, res) {
    if (res.status !== status) {
      const a = STATUS_CODES[status];
      const b = STATUS_CODES[res.status];
      return new Error('expected ' + status + ' "' + a + '", got ' + res.status + ' "' + b + '"');
    }
  }
Perform assertions on the response status and return an Error upon failure.

@param {Number} status
@param {Response} res
@return {?Error}
@api private