do not delete users_upgrade key after unsuccesful upgrade
do not delete users_upgrade key after unsuccesful upgrade

...because it is used for determining if repair is needed

Change-Id: I30913410cce0103bf80824ececbf61b42be4459c
Reviewed-on: http://review.couchbase.org/75555
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

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