CBQE-1: notify cache down but do not exit

Change-Id: I48e756055268098c6d728eedca5f38942d000cbf
Reviewed-on: http://review.couchbase.org/24566
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>