code: function _normalizeSuccess(onSuccess) {
              var _onSuccess = onSuccess;
              if (onSuccess.length < 2) {
                  _onSuccess = function (res, next) {
                      var result = onSuccess(res);
                      next(null, result);
                  };
              } else if (onSuccess.length === 2) {
                  _onSuccess = onSuccess;
              } else {
                  throw new Error('args length invalid');
              }
              return _onSuccess;
          }
docs: thenのコールバックをラップして引数を共通化します
  @memberof Bucks
  @instance
  @private
  @param {Function} onSuccess ex) onSuccess(res, next) | onSuccess(res) | onSuccess()
  @throws {Error} args length invalid
  @return {function} wrappedTask onSuccess(res, next)