Improve error message for SSL CA verification failure

Summary: cURL can raise a wide variety of errors when SSL negotiation fails, here's another one we saw in the wild. Provide a more tailored error message.

Test Plan: Partial; just ran futures to verify I didn't break anything. Properly testing this requires bringing up a webserver with a self-signed certificate which doesn't seem like a good use of time relative to the complexity of the change since I have only a vague idea of how to do it.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, epriestley

Maniphest Tasks: T936

Differential Revision: https://secure.phabricator.com/D1778