Big Switch: Catch exceptions in watchdog thread

Catch and log exceptions in the watchdog greenthread to prevent
them from stopping the periodic consistency checks.

Closes-Bug: #1330135
Change-Id: I6834c0fee0429bb72b8c61307be7bdca77f6de9b
(cherry picked from commit f89a6bd30b6f99bc39f266ae8d3880380379f8b9)
Merge "Big Switch: Catch exceptions in watchdog thread" into stable/icehouse