Improve exception handling in _process_router_update()

Improve exception handling in _process_router_update()

in _process_router_update() where _process_router_if_compatible()
method is called only RouterNotCompatibleWithAgent exception is handled.
In case any other (intermittent) exception happens inside
_process_router_if_compatible() (e.g. MessagingTimeout on fetching
external net from server) it results in a situation where agent completely
forgets about the router and continues working as usual while server
shows router hosted by agent.

This patch adds handler for broader exception there and set
fullsync = True like it's done earlier in _process_router_update()
when getting routers from server.

Change-Id: I5bfc540a926349848702100744f97c597b852604
Closes-Bug: #1423571