The message was hardcoded to indicate that the test wanted a nil error, even
though in some cases a specific error was wanted. This patch fixes the
message to print the wanted error.

Change-Id: Id86ea89d6f41f25bfa164acc50142ae8ff0ec410
GitHub-Last-Rev: c220374845667942c608c02afadff95443e2ec20
GitHub-Pull-Request: golang/go#26674
Reviewed-on: https://go-review.googlesource.com/126619Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>