public static function batchFinished($success, $results, $operations) {
    // For some reason imported sessions (offerings) got status 13 instead of 1.
    // So update them manually or dive into the code and find the bug.
    $query = \Drupal::database()->update('node_field_data');
    $query->fields(['status' => '1']);
    $query->condition('status', 13);
    $query->execute();

    if ($success) {
      drupal_set_message(t('Great success!'));
    }
    else {
      $error_operation = reset($operations);
      drupal_set_message(t('An error occurred while processing @operation with arguments : @args', [ '@operation' => $error_operation[0], '@args' => print_r($error_operation[0], TRUE) ]));
    }
  }