7 – How do I add the “error” class when a form validation fails?

I’m working with Drupal 7.56 and I am using the Webform module to create a custom form.

I used hook_form_alter() in a custom module to add a validation test.

function mymodule_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id=='webform_client_form_1') {
    $form('#validate')()='mymodule_form_validate';
    return $form;
  }
}

function mymodule_form_validate($form, &$form_state) {
  $aux = $form_state('values')('submitted')('aux');
  if ($aux != '') {
    if( /* my test */ ) {
      $text = t('Please check your input');
      form_set_error('aux', $text);
    }
  }
}

How can I add the error class when my test fails?