`mapreduce.Finish`中嵌套使用`mapreduce.MapReduce`会导致`Finish`变成非阻塞操作

嵌套的MapReduce的reducer中如果不调用writer.Write方法，会产生一个ErrReduceNoOutput错误，Finish 中 worker 返回异常会直接结束 Finish 调用，但是Finish中调用的MapReduceVoid会吞掉ErrReduceNoOutput错误返回一个 nil，从最后结果看是没有异常的成功调用，实际其他的 worker 都还在异步运行

例如下面这样的调用：

func main(){
        err := mapreduce.Finish(func() error {
		return worker1()
	}, func() error {
		val, err := mapreduce.MapReduce(func(source chan<- interface{}) {
			for i := 0;i<10;i++{
				source <- i
			}
		}, func(item interface{}, writer mapreduce.Writer, cancel func(error)) {
			i := item.(int)
			writer.Write(i * i)
		}, func(pipe <-chan interface{}, writer mapreduce.Writer, cancel func(error)) {
			var cnt int
			for i := range pipe{
				cnt += i.(int)
			}
                         // 这里不调用Write 会导致当前这个 worker 任务返回一个异常
			// writer.Write(cnt) 
		})
                 // 收到一个异常 `ErrReduceNoOutput`
		if err != nil {
			return err
		}
		fmt.Println("result:", val)
	})
       // 这里的 err 是 nil
       if err != nil {
           fmt.Println(err)
      }
}

Start by tracing Finish, MapReduce, and MapReduceVoid, focusing on how nested errors and worker completion are propagated. Reproduce the nested reducer case where ErrReduceNoOutput is swallowed; done means Finish reports the error and does not return while other workers are still running.