Also, fix some error messages.

Fixes #22065

Change-Id: Iac05c24b7bb128be3f43b8f2aa180b3957d5ee72
Reviewed-on: https://go-review.googlesource.com/66390
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>