DS18B20 reporting nan

Now that the DS18B20 plugin doesn't crash (since !31 (merged)), it instead returns the string nan to Home Assistant, which dutifully displays nan °F.

I suspect we need to return unknown or unavailable. Not sure which off the top of my head.

Worst case we can simply not return a state at all when the temperature is NaN, which will cause it to time out. But explicitly returning a failure is better IMO.