NCBC-325: MemcachedClient.ExecuteGet(...) does not return error codes

Change-Id: Icba87d8096cbf4444632c518e158571fbce54d66
Reviewed-on: http://review.couchbase.org/30718
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>