Catch exceptions in router rescheduler

Catch and log exceptions in router rescheduling loop
rather than just dying which would stop all future
router rescheduling attempts.

This prevents transient DB connectivity issues from
permanently breaking the rescheduler until the process
restarts.

Conflicts:

	neutron/db/l3_agentschedulers_db.py

Closes-Bug: #1375597
Change-Id: I2ab37847074fa6bbdd2b13fd03b8742996dcfc78