Arducopter: Create Vcc warning system.  It piggy-backs on the existing low battery warning event.  It will not do a FS event unless failsafe_battery_event = True.  It will still cause the lights to flash if that is enabled. This seems appropriate though obviously won't protect people if they don't enable it.

Could probably use more work such as a new ERROR_SUBSYSTEM_FAILSAFE for this specific problem.