Elgg\Ajax\Service::respondWithError PHP Method

    public function respondWithError($msg = '', $status = 400)
    {
        $response = new JsonResponse(['error' => $msg], $status);
        $this->response_sent = true;
        return _elgg_services()->responseFactory->send($response);
    }

 /**
  * Send error HTTP response
  *
  * @param string $error       Error message
  * @param int    $status_code HTTP status code
  * @param array  $headers     HTTP headers (will be discarded on AJAX requests)
  * @return Response
  * @throws InvalidParameterException
  */
 public function respondWithError($error, $status_code = ELGG_HTTP_BAD_REQUEST, array $headers = [])
 {
     if ($this->ajax->isReady()) {
         return $this->send($this->ajax->respondWithError($error, $status_code));
     }
     if ($this->isXhr()) {
         // xhr calls to non-actions (e.g. ajax/view or ajax/form) need to receive proper HTTP status code
         return $this->send($this->prepareResponse($error, $status_code, $headers));
     }
     $forward_url = $this->request->headers->get('Referer');
     if (!$this->isAction()) {
         $params = ['current_url' => current_page_url(), 'forward_url' => $forward_url];
         // For BC, let plugins serve their own error page
         // @see elgg_error_page_handler
         $forward_reason = (string) $status_code;
         $forward_url = $this->hooks->trigger('forward', $forward_reason, $params, $forward_url);
         if ($this->response_sent) {
             // Response was sent from a forward hook
             return $this->response_sent;
         }
         $params['type'] = $forward_reason;
         $error_page = elgg_view_resource('error', $params);
         return $this->send($this->prepareResponse($error_page, $status_code));
     }
     $forward_url = elgg_normalize_url($forward_url);
     return $this->send($this->prepareRedirectResponse($forward_url));
 }