protected static function get_error_filename($statusCode, $instance = null)
    {
        if (!$instance) {
            $instance = ErrorPage::singleton();
        }
        // Allow modules to extend this filename (e.g. for multi-domain, translatable)
        $name = "error-{$statusCode}.html";
        $instance->extend('updateErrorFilename', $name, $statusCode);
        return $name;
    }