Memcached hasKey should test for success, the get can fail for other reasons.

One of the other failures is no running server.
Merge pull request #8512 from owncloud/fix-memcached-hasKey

Be more strict in checking the resultCode of memcached hasKey function

* owncloud/fix-memcached-hasKey:
  Memcached hasKey should test for success, the get can fail for other reasons.