multiGet: handle 404s less noisy

carbonserver returns 404s for metrics that don't exist, instead of an
empty result.  This is easier for both carbonserver as well as us
(carbonzipper) because it means no pickle has to be constructed, as well
as no pickle has to be unpacked, let alone an empty array examined.
Merge pull request #1 from grobian/master

multiGet: handle 404s less noisy