vet: clean up taglit error print when typechecking fails.
vet: clean up taglit error print when typechecking fails.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7416050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go