Directly return error instead of assigning to err and then returning.

Change-Id: Ie5c466cac70cc6d52ee72ebba3e497e0da8a5797
Reviewed-on: https://go-review.googlesource.com/73531
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>