change switch_core_timer_next to return switch_status_t, update all users to new usage, add some error checking to mod_conference so that it bails out of the loops more gracefully on shutdown.

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@4454 d0543943-73ff-0310-b7d9-9358b9ac24b2