Add debug message if a callback function is set but does not exist

Really really great and helpful module, thank you very much!

I just had the situation where I wondered why the field did not appear in views and the reason was a typo in the callback function name.

It's good that you check its existence, but if the callback function is set but not existing a warning message would help a lot. This would also show if the file is not correctly included for example.
In my eyes it would not make sense to add a callback function that does not exist.

Related code:

/**
 * Helper function to get all extra field for an entity type.
 *
 * @param string $entity_type
 *   Name of the entity.
 * @param array $bundles
 *   All bundles of the entity type.
 * @param array $extra_fields
 *   Reference for the extra fields array.
 */
function _extrafield_views_integration_get_extra_fields_for_entity_type($entity_type, $bundles, &$extra_fields) {
  foreach ($bundles as $bundle) {
    $temp_fields = field_info_extra_fields($entity_type, $bundle, 'display');
    if (!empty($temp_fields)) {
      foreach ($temp_fields as $field_name => $temp_field) {
        if (isset($temp_field['callback'])) {
          if (isset($temp_field['file']) && file_exists($temp_field['file'])) {
            require_once( DRUPAL_ROOT . '/' . $temp_field['file']);
          }
          if (function_exists($temp_field['callback'])) {
            $extra_fields[$field_name] = array(
              'entity_type' => $entity_type,
              'label' => $temp_field['label'],
              'description' => $temp_field['description'],
              'callback' => $temp_field['callback'],
            );
            if (isset($temp_field['file'])) {
              $extra_fields[$field_name]['file']
                = DRUPAL_ROOT . '/' . $temp_field['file'];
            }
          }
        }
      }
    }
  }
}