While processing a router update in _process_router_update method,
if an exception occurs, we try to do a full_sync.
We only need to re-sync the router whose update failed.
Addressed a TODO in the same method, which falls in similar lines.
Change-Id: I7c43a508adf46d8524f1cc48b83f1e1c276a2de0
Closes-Bug: #1494682