Remove noise logs in tablet move

[all-seeing-code]

When move tablets are running to different groups the whole stack trace can show up in the Zero log, which doesn't kill the zero but makes it look like an issue. This change edits the message to prevent logging the stack trace.

Zero Logs (Before):

E0429 16:41:56.818326     340 raft.go:352] While applying tablet proposal: Tablet is already being served
github.com/dgraph-io/dgraph/dgraph/cmd/zero.init
	/home/alvis/go/src/github.com/dgraph-io/dgraph/dgraph/cmd/zero/raft.go:161
runtime.doInit
	/usr/local/go/src/runtime/proc.go:5414
runtime.doInit
	/usr/local/go/src/runtime/proc.go:5409
runtime.doInit
	/usr/local/go/src/runtime/proc.go:5409
runtime.doInit
	/usr/local/go/src/runtime/proc.go:5409
runtime.doInit
	/usr/local/go/src/runtime/proc.go:5409
runtime.main
	/usr/local/go/src/runtime/proc.go:190
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1373

Zero Logs (After)

E0429 17:44:14.433879    3184 raft.go:352] While applying tablet proposal: Tablet is already being served
E0429 17:44:14.433918    3184 raft.go:707] While applying proposal: Tablet is already being served

Fixes: DGRAPH-1141

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This change is 

[martinmr]

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @manishrjain)

[manishrjain]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ashish-goswami)