Add stuck key detection (temporary until diodes are added to keypad board or held key detection is added)

For now, if a key is stuck for some reason, the firmware won't detect any more keys, and the keypad will appear "stuck", responding to I2C requests but not responding to keypresses. There needs to be some way to detect a stuck key, either by I2C request or by special response code that could be detected in the firmware - so that the UI software could at least show a helpful error message.