Improve SSL error message for cURL error 51 (CURLE_SSL_PEER_CERTIFICATE / CURLE_PEER_FAILED_VERIFICATION)

Summary:
If you self-sign a certificate and follow the instructions, you may end up getting hit with this because you signed the cert for the wrong domain. Provide better error text.

See: https://github.com/facebook/phabricator/issues/98

Test Plan: Self-signed a certificate, installed it in apache, much song and dance, resolved the error, got a valid self-signed CA working.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1857