How can you prevent a Webform from going to a confirmation modal if remote post handler fails in a service response?
I believe it is too late to halt the Webform with validation errors once it is past the validation phase ie. in postSave(). If you are expecting validation errors on user input however, then you could move the service call to WebformHandlerBase::validateForm() and then save the service response in the $form_state object if you need it.
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &amp;$form, FormStateInterface $form_state, WebformSubmissionInterface $webform_submission) {
    parent::validateForm($form, $form_state, $webform_submission);

    try {
      $response = $this-&gt;doServiceRequest()
    }
    catch (\Exception $e) {
      $form_state-&gt;setErrorByName('foo', $e-&gt;getMessage());
      return;
    }

    // Save service response output.
    $webform_submission-&gt;setData($response-&gt;getData() + $webform_submission-&gt;getData());
  }

Note: This probably goes against the design of the RemotePostWebformHandler, but I think it could work..., or you could directly extend WebformHandlerBase instead.