cmd/go: update error messages in tests to match CL 332573

cmd/go: update error messages in tests to match CL 332573

I neglected to run the 'longtest' builder, and the tests
that cover the error message changed in CL 332573 apparently
do not run in short mode.

Updates #36460
Updates #42661

Change-Id: I53500ddaca8ac9f0dfaab538923b3c9a4f71665e
Reviewed-on: https://go-review.googlesource.com/c/go/+/334889
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>