Source-code Merge Request #14: backend_canonselphyneo: Improve printer state handling (open)

When an error like "No Ink" or "No paper tray" on a canon selphy occurs (I have a CP1300), it currently stops the printer in cups. However, the printer finishes the job when the source of the error is cleared (e.g.: replaced ink cartridge).

I change the error handling so that the backend waits for these transient errors to be resolved and then continues operation normally instead of stopping the printer.
This should better reflect the behaviour of the Selphy printer.