getFormAjaxException example

$this->messenger->addError($this->t('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@size) that this server supports.', ['@size' => $this->formatSize($exception->getSize())]));
      $response = new AjaxResponse(NULL, 200);
      $status_messages = ['#type' => 'status_messages'];
      $response->addCommand(new PrependCommand(NULL, $status_messages));
      $event->allowCustomResponseCode();
      $event->setResponse($response);
      return;
    }

    // Extract the form AJAX exception (it may have been passed to another     // exception before reaching here).     if ($exception = $this->getFormAjaxException($exception)) {
      $request = $event->getRequest();
      $form = $exception->getForm();
      $form_state = $exception->getFormState();

      // Set the build ID from the request as the old build ID on the form.       $form['#build_id_old'] = $request->request->get('form_build_id');

      try {
        $response = $this->formAjaxResponseBuilder->buildResponse($request, $form, $form_state, []);

        // Since this response is being set in place of an exception, explicitly