Customising/Overriding the _uc_out_of_stock_get_error() function

Sorry that I took a while to answer. You are right about pointing out that giving a way to override that function is useful. I might change it to a theme_xx() function sometime in the future. Thing is that the function handles different error messages depending on the error and if the product has attributes or not.

There is, however, something you can do. Those errors are enclosed with t() for translation. I use this trick quite a bit to modify this kind of strings, which is by enabling the locale module and create a new language (EN-special). By doing that you can 'translate' any string into whatever you want, and by translate it doesn't necessary mean another language, just another string. Strings that are not 'translated' would be displayed as default.

The attached patch allows other modules to extend / alter the error message.

I, for example, use something like this (in a custom module) :

 /**
 * Implements hook_uc_out_of_stock_error_alter().
 * 
 * @param array $error
 * @param object $product 
 */
function uc_cart_alter_uc_out_of_stock_error_alter(&$error, $product) {
  switch ($error['type']) {
    case 'not_enough':
      $error['msg'] = t("We're sorry. We have now only @qty of the product @product left in stock.", array('@qty' => format_plural($error['stock'], '1 unit', '@count units'), '@product' => $error['product']));
      // Clearly state the desired action
      $error['msg'] .= ' '. t("Adjust your <a href=\"@cart-url\">shopping cart</a> accordingly.", array('@cart-url' => url('cart')));
      break;

    case 'out_of_stock':
      // Clearly state the desired action
      $error['msg'] .= ' '. t("Adjust your <a href=\"@cart-url\">shopping cart</a> accordingly.", array('@cart-url' => url('cart')));
      if (module_exists('uc_stock_notify')) {
        // Give an option te get notified once back in stock
        $error['msg'] .= ' '. t("<a href=\"@stock_notify_url\">Notify when back in stock.</a>", array('@stock_notify_url' => url('stock_notify/'. $product->model, array('query' => drupal_get_destination()))));
      }
      break;
  }
} 

nice way of doing it. thanks.