Club Drupal

 *   The array of errors (keyed by field name, language code, and delta) that

 *   have already been reported for the entity. The function should add its

 *   errors to this array. Each error is an associative array with the following

 *   keys and values:

 *   - error: An error code (should be a string prefixed with the module name).

 *   - message: The human readable message to be displayed.

  // 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.'),

          );

        }

      }

    }

  }