local backend: worker error is eaten on some case

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

suppose when execute job we return the remaining storage capacity of TiKV(%s) is less than 10%%, the context will be cancelled

tidb/br/pkg/lightning/backend/local/local.go

Lines 1675 to 1679 in d74298c

	for i := 0; i < local.WorkerConcurrency; i++ {
	workGroup.Go(func() error {
	return local.startWorker(workerCtx, jobToWorkerCh, jobFromWorkerCh, &jobWg)
	})
	}

then here we noticed that context cancelled and return context cancelled error(might be wrapped inside other error, such as rpc error: code = Canceled desc = context canceled: [BR:PD:ErrPDBatchScanRegion]batch scan region" during page scan regions)

tidb/br/pkg/lightning/backend/local/local.go

Line 1683 in d74298c

err := local.prepareAndSendJob(

and in here, we record this context cancelled err, not the err that causes context cancel is eaten

tidb/br/pkg/lightning/backend/local/local.go

Lines 1692 to 1697 in d74298c

	if err != nil {
	firstErr.Set(err)
	workerCancel()
	_ = workGroup.Wait()
	return firstErr.Get()
	}

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master