How to show error on screen after failling a webservice call on Webform submitForm?

I made a custom handler for one of my webform to call a service when the form is submitted. Here is my code. It is working when the service return a response with status code = 200 but I cannot find a way to notify the view when an error is raised by the webservice. I would like to show a modal with an error message but cannot find how to do this.

class CRMWebformHandler extends WebformHandlerBase {

  public function submitForm(array &$form, FormStateInterface $form_state, WebformSubmissionInterface $webform_submission) {
    try {
      $data = array(
        'firstName' => $webform_submission->getData()['lastname'],
        'lastName' => $webform_submission->getData()['firstname'],
        'civility' => $webform_submission->getData()['civility'],
        'email' => $webform_submission->getData()['email'],
      );
     
      $payload = json_encode($data);
      $curl = curl_init();
      curl_setopt($curl, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
      curl_setopt($curl, CURLOPT_POST, 1);
      curl_setopt($curl, CURLOPT_POSTFIELDS, $payload);
      curl_setopt($curl, CURLOPT_URL, "https://my-url.com");
      curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);

      $result = curl_exec($curl);
      curl_close($curl);

      $json = json_decode($result, TRUE);
      $succeed = is_bool($json) ? $json : FALSE;
      if (!$succeed && curl_getinfo($curl, CURLINFO_HTTP_CODE) !== 200) {
        throw new \Exception('No valid response from server.');
      }
    }
    catch (\Exception $exception) {
      \Drupal::messenger()->addError('No valid response from server.');
      return;
    }
  }
}