code: 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);
                  }
              });
          }
docs: 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}