code: protected function processAlerts()
      {
          $errorSummary = $this->errorSummary->build($this->debug->internal->errorStats());
          if ($errorSummary) {
              \array_unshift($this->data['alerts'], array(
                  'alert',
                  array($errorSummary),
                  array(
                      'class' => 'danger error-summary',
                      'dismissible' => false,
                  )
              ));
          }
          return parent::processAlerts();
      }
docs: process alerts
  
  @return string