do not fail if resolve lock returns region unavailable

[vivid392845427]

What problem does this PR solve?

the testcase return error message as follow，because nemesis：pd-scheduler random-merge-scheduler

2021/09/07 05:23:35 pd_scheduler.go:59: [info] apply nemesis pd-scheduler random-merge-scheduler on ns tipocket-debug-nrrdr 2021/09/07 05:23:35 pd_scheduler.go:63: [info] add scheduler random-merge-scheduler successfully 2021/09/07 05:27:34 resolve_lock.go:233: [error] [round-35] failed to run GC at safe point 427552777833021443 2021/09/07 05:27:34 resolve_lock.go:194: [info] test end 2021/09/07 05:27:34 control.go:285: [fatal] run client error, GET request "http://debug-tidb.tipocket-debug-nrrdr.svc:10080/test/gc/resolvelock?safepoint=427552777833021443&physical=false", got 400 resolveLocks failed: region unavailable

What is changed and how does it work?

none

Check List

Tests

• Unit test
• E2E test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has Go code change
• Has CI related scripts change
• Has Terraform scripts change

Side effects

• Breaking backward compatibility

Related changes

• Need to update the documentation

Does this PR introduce a user-facing change?:

NONE