Harden UUID parsing

If the scheduler sees a UUID that is not formatted properly,
it should ignore the message as invalid.

Change-Id: I4aff1cd035bc80902dfc37f693f33aad6cdc146d
protect against None in target

Change-Id: I672417fdf389a0df69fc3de9431dabe5852710f3
Merge pull request #146 from dhellmann/harden-uuid-handling

Harden UUID parsing