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/44371Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/buildtag.go

@@ -52,6 +52,7 @@ func checkBuildTag(name string, data []byte) {
 			if !bytes.Equal(fields[0], plusBuild) {
 				// Comment is something like +buildasdf not +build.
 				fmt.Fprintf(os.Stderr, "%s:%d: possible malformed +build comment\n", name, i+1)
+				setExit(1)
 				continue
 			}
 			if i >= cutoff {
@@ -85,6 +86,7 @@ func checkBuildTag(name string, data []byte) {
 		// Comment with +build but not at beginning.
 		if bytes.Contains(line, plusBuild) && i < cutoff {
 			fmt.Fprintf(os.Stderr, "%s:%d: possible malformed +build comment\n", name, i+1)
+			setExit(1)
 			continue
 		}
 	}