The old one was rather confusing - it makes it sound like the user has done something wrong.

Change-Id: Ibc7411f4f1d5f6c66fbcaac64bb05b0743354418
GitHub-Last-Rev: 09290accddb23848ee80d641e4f2bcf6ef686e01
GitHub-Pull-Request: golang/go#27979
Reviewed-on: https://go-review.googlesource.com/c/139102Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>