Check for cookie_sync return value when leaving cluster.
Check for cookie_sync return value when leaving cluster.

Currently it cannot return anything but {ok, Cookie}, but I find it
clearer to check for return value anyway. And we do this in other
places.

Change-Id: I0dd51e7cd3a5569c845025bacb985b8247670468
Reviewed-on: http://review.couchbase.org/69043
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server