acme: improve TLSSNI{01,02}ChallengeCert methods

This is a split of https://go-review.googlesource.com/23970 (patch set 8)
to address only Client changes:

1. Expose expected server name value of TLS SNI ClientHello message
2. Fix a bug where returned error value was nil.

Change-Id: I21f571652e9bbef80a2222dc34fce767270b7c48
Reviewed-on: https://go-review.googlesource.com/26852
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed