code: public function onError(Event $event)
      {
          if ($event['inConsole'] || !$event['isFirstOccur']) {
              return;
          }
          $this->processLogEntry(
              'errorNotConsoled',
              array(
                  $event['typeStr'].': '.$event['file'].' (line '.$event['line'].'): '.$event['message']
              ),
              array(
                  'channel' => 'phpError',
                  'class' => $event['type'] & $this->debug->getCfg('errorMask')
                      ? 'danger'
                      : 'warning',
              )
          );
      }
docs: errorHandler.error event subscriber
  
  Used to capture errors that aren't sent to log (ie debug capture is false)
  
  @param Event $event event object
  
  @return void