MINOR: connection: add a new error code for SSL with heartbeat

MINOR: connection: add a new error code for SSL with heartbeat

Users have seen a huge increase in the rate of SSL handshake failures
starting from 2014/04/08 with the release of the Heartbleed OpenSSL
vulnerability (CVE-2014-0160). Haproxy can detect that a heartbeat
was received in the incoming handshake, and such heartbeats are not
supposed to be common, so let's log a different message when a
handshake error happens after a heartbeat is detected.

This patch only adds the new message and the new code.