MB-8418 Handle stats key exceptions according to memcached errors

Change-Id: Ibfe2c046b20f6913184dbc8a251d71451f5345d7
Reviewed-on: http://review.couchbase.org/26808
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>