cmd/vet: set exit status to non zero for all cases

cmd/vet: set exit status to non zero for all cases

Vet returns with a nonzero exit for all possible messages in the
buildtag check. However for this file:

    //+buildlinux

    package main

vet returns a zero exit status:

    $ go vet main.go
    demo.go:1: possible malformed +build comment
    $ echo $?
    0

This CL sets the exit status to non zero for the remaining messages in
the buildtag check.

Change-Id: Ia2c35ebc3ec5ac311d2a0295b5b9fdd997a85726
Reviewed-on: https://go-review.googlesource.com/44371
Reviewed-by: Rob Pike <r@golang.org>