update_manager_download_batch_finished

update_manager_download_batch_finished($success, $results)

function update_manager_download_batch_finished($success, $results) {
  if (!empty($results['errors'])) {
    $item_list = array(
      '#theme' => 'item_list',
      '#title' => t('Downloading updates failed:'),
      '#items' => $results['errors'],
    );
    drupal_set_message(drupal_render($item_list), 'error');
  }
  elseif ($success) {
    drupal_set_message(t('Updates downloaded successfully.'));
    $_SESSION['update_manager_update_projects'] = $results['projects'];
    return new RedirectResponse(\Drupal::url('update.confirmation_page', [], ['absolute' => TRUE]));
  }
  else {
    // Ideally we're catching all Exceptions, so they should never see this,
    // but just in case, we have to tell them something.
    drupal_set_message(t('Fatal error trying to download.'), 'error');
  }
}