Add exception catch in report_state for DBError

Add exception catch in report_state for DBError

We discovered while testing Cinder in an HA
environment that transient DB errors can be
encountered that are not currently covered by
the exception catch in service.py report_state().
The uncaught exceptions were causing the thread
for report_state to prematurely exit and causing
services to no longer update the DB when, in fact,
they were still usable.

This change adds an exception catch for DBError
so that the thread can continue to function.

Change-Id: I95f79b8d6c8f5d7b3e44665306b500b8d2ce3c7c
Closes-bug: 1466991