See https://go-review.googlesource.com/#/c/38313/ for background.
It turns out that only a few tests checked for this.

The new error message is shorter and very clear.

Change-Id: I8ab4ad59fb023c8b54806339adc23aefd7dc7b07
Reviewed-on: https://go-review.googlesource.com/38314
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>