public static function error( $fieldKey, $template = null )
    {

        if (!session_id()) {
            session_start();
        }

        if (isset( $_SESSION[ static::SESSION_DATA_KEY ] )) {
            if (count($_SESSION[ static::SESSION_DATA_KEY ]) > 0) {
                self::$errors = $_SESSION[ static::SESSION_DATA_KEY ];
                unset( $_SESSION[ static::SESSION_DATA_KEY ] );
            }
        }

        if (isset( self::$errors[ $fieldKey ] )) {
            $message = self::$errors[ $fieldKey ];
            if (!is_null($template)) {
                $message = str_ireplace(":message", $message, $template);
            }

            return $message;
        }

        return "";
    }