[br] br error message not accurate when pd system time incorrect

[fubinzh]

Bug Report

Please answer these questions before submitting your issue. Thanks!

• pd time not accurate
• start br restore

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

• br error message should be clear

3. What did you see instead (Required)

• br error message is confusing, currently error message is like this, it is not accurate
  Error: rpc error: code = Unknown desc = rpc error: code = Unavailable desc = not leader

$ grep ERROR pd.log
[2021/10/20 03:34:56.883 +00:00] [ERROR] [middleware.go:104] ["redirect but server is not leader"] [from=downstream-pd-0] [server=downstream-pd-0] [error="[PD:apiutil:ErrRedirect]redirect failed"]
[2021/10/20 03:59:53.096 +00:00] [ERROR] [middleware.go:104] ["redirect but server is not leader"] [from=downstream-pd-0] [server=downstream-pd-0] [error="[PD:apiutil:ErrRedirect]redirect failed"]
[2021/10/20 03:59:53.108 +00:00] [ERROR] [tso.go:207] ["system time may be incorrect"] [last=2021/10/20 03:59:53.231 +00:00] [next=2021/10/20 03:59:53.108 +00:00] [error="[PD:common:ErrIncorrectSystemTime]incorrect system time"]

$ grep ERROR br.log.2021-10-20T04.00.09Z
[2021/10/20 04:00:13.725 +00:00] [ERROR] [client.go:600] ["[pd] getTS error"] [dc-location=global] [error="PD:client:ErrClientGetTSO]rpc error: code = Unknown desc = [PD:tso:ErrGenerateTimestamp]generate timestamp failed, requested pd is not leader of cluster: rpc error: code = Unknown desc = [PD:tso:ErrGenerateTimestamp]generate timestamp failed, requested pd is not leader of cluster"] [stack="github.com/tikv/pd/client.(*client).handleDispatcher\n\t/nfs/cache/mod/github.com/tikv/pd@v1.1.0-beta.0.20210818082359-acba1da0018d/client/client.go:600"]
[2021/10/20 04:00:13.978 +00:00] [ERROR] [client.go:600] ["[pd] getTS error"] [dc-location=global] [error="[PD:client:ErrClientGetTSO]rpc error: code = Unknown desc = [PD:tso:ErrGenerateTimestamp]generate timestamp failed, requested pd is not leader of cluster: rpc error: code = Unknown desc = [PD:tso:ErrGenerateTimestamp]generate timestamp failed, requested pd is not leader of cluster"] [stack="github.com/tikv/pd/client.*client).handleDispatcher\n\t/nfs/cache/mod/github.com/tikv/pd@v1.1.0-be
ta.0.20210818082359-acba1da0018d/client/client.go:600"]
[2021/10/20 04:00:14.188 +00:00] [ERROR] [client.go:600] ["[pd] getTS error"] [dc-location=global] [error="PD:client:ErrClientGetTSO]rpc error: code = Unknown desc = [PD:tso:ErrGenerateTimestamp]generate timestamp failed, requested pd is not leader of cluster: rpc error: code = Unknown desc = [PD:tso:ErrGenerateTimestamp]generate timestamp failed, requested pd is not leader of cluster"] [stack="github.com/tikv/pd/client.*client).handleDispatcher\n\t/nfs/cache/mod/github.com/tikv/pd@v1.1.0-beta.0.20210818082359-cba1da0018d/client/client.go:600"]