do not crash ns_memcached on get_keys -> {memcached_error, _}
do not crash ns_memcached on get_keys -> {memcached_error, _}

Change-Id: I7bec4078a2015ee6528a677ccc65b5331597c0e8
Reviewed-on: http://review.couchbase.org/51841
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

ceejatec/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,nimishzynga/ns_server,mschoch/ns_server,nimishzynga/ns_server