Use an early return. Check errors. Deduplicate.

Change-Id: Iabefd563b5ef82a16fab4791277630804fd09003
Reviewed-on: https://go-review.googlesource.com/20597
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>