Keep track of node-task assignements

[jdupl]

Prevent refusal-proposition loop if an unknown (for now) exception occurs.

• No ffmpeg
• no more local disk space
• Network disconnected
• libav<->ffmpeg problem

The double assignment protection (concurrency) is not affected by this.

Solution is to never assign n times the same job (not task) to a node.

• Perhaps retry from time to time (network outage)
• Try to detect if problem is temporary
• Crash report object ?