If printer not connected, program should wait until it is connected

[sberl]

Current behavior is for the program to exit immediately if the printer is off or not connected. Instead it should display a message telling the user to connect and power on the printer, and then wait until that has been done, or some sort of quit key is pressed.

This will facilitate making the program autostart at boot of the raspberry pi, so that no keyboard or mouse will be required to operate the kiosk.

Implementation will involve catching the USBNotFoundError exception.