vcgencmd on Raspberry Pi sometimes returns -1

[coogle]

I have a Raspberry Pi 3 (with the 7" touchscreen) which breaks the script on line 133 (roughly) where you try to extract display_state and don't expect display_power=-1 as a possible return value.

I fixed this locally by just wrapping that line in a try/catch and returning "Unknown" for this sensor, but a better patch is probably a good idea.