Readings <22cm are being transmitted

[crouchingbadger]

Since we've installed more v4.1 sensors the number of spikes (peaks caused by low readings) has massively increased. Readings <22cm are below the minimum distance and should not be transmitted. Instead, consider an error message.

Debugging would also be useful to see

1. if all the reads on a particular wakeup period are < 22cm
2. the potential causes of this. Why does it fail to read?

Also consider modifying Nominet bridge code to reject or annotate errors.