cmd/bundle: More idiomatic flag.Usage.

The responsibility of flag.Usage is to print usage to stderr:

> Usage prints to standard error a usage message documenting all
> defined command-line flags.

Calling os.Exit(2) is outside of its scope, flag package does that.

Change-Id: I3eef2b796e5ddedff6d5927dbcdbfff3a6082270
Reviewed-on: https://go-review.googlesource.com/23788
Reviewed-by: Joseph Holsten <joseph@josephholsten.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

cmd/bundle/main.go

@@ -129,7 +129,6 @@ func addImportMap(s string) {
 func usage() {
 	fmt.Fprintf(os.Stderr, "Usage: bundle [options] <src>\n")
 	flag.PrintDefaults()
-	os.Exit(2)
 }
 
 func main() {
@@ -141,6 +140,7 @@ func main() {
 	args := flag.Args()
 	if len(args) != 1 {
 		usage()
+		os.Exit(2)
 	}
 
 	if *dstPath != "" {