better failover when no snapshot created closes #47

Small PR fixes issues when no snapshot was created at all - and lolcommits blows up on the resizing.  

If now snapshot was created from the capture, the command now exits with 1 (return code) and has the following message in the debug output;

`Runner: failed to capture a snapshot`

This closes issue #47, to fail better if the camera goes missing (or when a bad `LOLCOMMITS_DEVICE` has been specified).

I also cleaned up some of the methods calls in `def run`.