[NCBC-555] Multi get can fail and return a null StatusCode

Change-Id: Ie2fd1d9d62c04c04e54af4b8fc89b52c363f437b
Reviewed-on: http://review.couchbase.org/39658
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>