Prevent Phabot from spinning out of control

Summary: When the remote closes the connection, phabot goes into a busy loop
because of PHP's "nothing should ever be an error" semantics. Instead, detect
connection termination.
Test Plan: Disabled the "PONG" response in the protocol handler and let freenode
disconnect phabot. It spun out of control before, now it detects the issue and
exits to await automatic restart.
Reviewed By: jungejason
Reviewers: codeblock, jungejason, aran, tuomaspelkonen
CC: aran, jungejason
Differential Revision: 582