前のタスクがエラーなく成功した場合のみ呼ばれるタスクを追加します
@memberof Bucks
@instance
@param {Function} onSuccess `onSuccess(res, next)`. `next(err, res)`
@throws {Error} args length invalid
@return {Bucks}
function then(onSuccess) {
            var _onSuccess = this._normalizeSuccess(onSuccess);
            return this.add(function onSuccessTask(err, res, next) {
                if (err) {
                    // pass this error to next task
                    next(err);
                } else {
                    // call onSuccess
                    _onSuccess(res, next);
                }
            });
        }