/**
 * Implements hook_field_attach_validate().
 */
function {{ machine_name }}_field_attach_validate($entity_type, $entity, &$errors) {
  // Make sure any images in article nodes have an alt text.
  if ($entity_type == 'node' && $entity->type == 'article' && !empty($entity->field_image)) {
    foreach ($entity->field_image as $langcode => $items) {
      foreach ($items as $delta => $item) {
        if (!empty($item['fid']) && empty($item['alt'])) {
          $errors['field_image'][$langcode][$delta][] = array(
            'error' => 'field_example_invalid',
            'message' => t('All images in articles need to have an alternative text set.'),
          );
        }
      }
    }
  }
}