Remove broad exception catch from periodic_sync_routers_task

Remove broad exception catch from periodic_sync_routers_task

Although this change removes a broad exception from
periodic_sync_routers_task (neutron.agent.l3_agent), the
implementation still ensures if an exception --caught or uncaught --
prevents a call to the method to disable the fullsync, then the next
call to the method will perform a fullsync again.

Author:         Cedric Brandily <zzelle@gmail.com>
Co-Authored-By: Carl Baldwin <carl.baldwin@hp.com>
Change-Id: I6281886f3334100a18952578250c8154a0ed15a9