cmd/bundle: More idiomatic flag.Usage.
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 <461476587780aa9fa5611ea6dc3912c146a91760@josephholsten.com>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools