server: return nil when server is closed

[rleungx]

What problem does this PR solve?

Currently, when we stop PD, it will print the error stack which is confused even if there is only one PD.

2018/12/26 05:10:45.332 leader.go:116: [error] campaign leader err server closed
github.com/pingcap/pd/server.(*Server).campaignLeader
	/.../pd/server/leader.go:287
github.com/pingcap/pd/server.(*Server).leaderLoop
	/.../pd/server/leader.go:115
runtime.goexit
	/usr/local/go1.11.1/src/runtime/asm_amd64.s:1333

What is changed and how it works?

This PR fixes this problem. It will returns nil rather than an error when the server is closed in campaignLeader's loop.

Tests

• Manual test

[siddontang]

LGTM