Also replaces verbs for error message from %s to %v. In general, low
level IO APIs return an error value containing non-string types and
there's no guarantee that all the types implement fmt.Stringer
interface.

Change-Id: I8a6e2a80d5c721c772a83b9556bac16556eaa771
Reviewed-on: https://go-review.googlesource.com/73931
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>