lightning may return "context cancel" error when TiKV disk is slow

[lance6716]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

see UT of linked PR

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

error that indicate the TiKV space is not enough

3. What did you see instead (Required)

[2023/06/16 12:42:17.677 +08:00] [ERROR] [backend.go:353] ["import failed"] [engineTag=`test`.`t`:5] [engineUUID=09383b9a-7e8f-52d1-b6f1-ec86abee1258] [retryCnt=0] [takeTime=668.943958ms] [error="scan regions from start-key:7480000000000000FF5A5F728000000122FF9627480000000000FA, err: rpc error: code = Canceled desc = context canceled: [BR:PD:ErrPDBatchScanRegion]batch scan region"]

4. What is your TiDB version? (Required)