This matches exposing CertificateInvalidError.Cert.
and (exposing but not the spelling of) HostnameError.Certificate.

Fixes #13519.

Change-Id: Ifae9a09e063d642c09de3cdee8a728ff06d3a5df
Reviewed-on: https://go-review.googlesource.com/32644
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>