Inspect the crypto/tls error to recognize this case and give a more
helpful error.

Fixes #11111.

Change-Id: I63f6af8c375aa892326ccccbd29655d54d68df0b
Reviewed-on: https://go-review.googlesource.com/16079
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>