Error doesn't take a format string and appends its own newline. Phrase
the error like the other ones.

Change-Id: Ic3af857e5d4890207c74a6eb59a0d1067b503e1b
Reviewed-on: https://go-review.googlesource.com/16420Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>