Gracefully handle missing firmware UUID

[fhunleth]

See #16 for failure log.

A missing firmware UUID is a pretty bad situation, but we shouldn't crash. I think the right answer is to connect to NervesHub anyway and provide an empty UUID. Given how rare this situation should be in deployed devices, it seems fine for NervesHub to just log that the device is running unknown firmware. We may eventually want to force a firmware download to the device for a last-ditch attempt to remotely recover it. However, no remote recovery is possible if the device doesn't connect.