Heartbeat:
- Pass the actual error to the jQuery event 'heartbeat-connection-lost'.
- When in error state, keep trying to connect even when no response is expected until the error is cleared.
See #23216.

git-svn-id: http://core.svn.wordpress.org/trunk@24577 1a063a9b-81f0-0310-95a4-ce76da25c4cd