bucks.js
function _normalizeError(onError) {
            var _onError = onError;
            if (onError.length < 2 || 3 < onError.length) {
                throw new Error(onError.name + ': args length invalid. should be: onError(err, next)');
            }
            return _onError;
        }
function _normalizeError(onError) {
            var _onError = onError;
            if (onError.length < 2 || 3 < onError.length) {
                throw new Error(onError.name + ': args length invalid. should be: onError(err, next)');
            }
            return _onError;
        }
error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} onError ex) onError(err, next)
@throws {Error} args length invalid
@return {function} wrappedTask ex) onError(err, next)