ugly error log

[siddontang]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

Just start PD

2. What did you expect to see?

[2019/04/29 07:49:49.687 +00:00] [WARN] [history_buffer.go:137] ["load history index failed"] [error="leveldb: not found",errorVerbose="leveldb: not found\ngithub.com/pingcap/pd/server/core.(*leveldbKV).Load\n\t/go/src/github.com/pingcap/pd/server/core/levedb_kv.go:40\ngithub.com/pingcap/pd/server/region_syncer.(*historyBuffer).reload\n\t/go/src/github.com/pingcap/pd/server/region_syncer/history_buffer.go:135\ngithub.com/pingcap/pd/server/region_syncer.newHistoryBuffer\n\t/go/src/github.com/pingcap/pd/server/region_syncer/history_buffer.go:54\ngithub.com/pingcap/pd/server/region_syncer.NewRegionSyncer\n\t/go/src/github.com/pingcap/pd/server/region_syncer/server.go:87\ngithub.com/pingcap/pd/server.newRaftCluster\n\t/go/src/github.com/pingcap/pd/server/cluster.go:75\ngithub.com/pingcap/pd/server.(*Server).startServer\n\t/go/src/github.com/pingcap/pd/server/server.go:230\ngithub.com/pingcap/pd/server.(*Server).Run\n\t/go/src/github.com/pingcap/pd/server/server.go:306\nmain.main\n\t/go/src/github.com/pingcap/pd/cmd/pd-server/main.go:110\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:200\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_amd64.s:1337"]

3. What did you see instead?

As you can see, the message contains stack and it is ugly and not useful, we should remove the stack and keet the message clean.

4. What version of PD are you using (pd-server -V)?

Release Version: v3.0.0-beta.1-26-g67549be8-dirty
Git Commit Hash: 67549be8b94e2465949de0a88ab07d0abb75abd0
Git Branch: master
UTC Build Time:  2019-04-29 02:56:24