Handle safety interruption informatively: temperature sensor safety trigger

[ErikDeBruijn]

If the temperature sensor has a problem (e.g. it reads too low for too long while the heater is on), it should interrupt the build and turn of the extruder heater. Heating up and resuming a build should be possible... The triggered interrupt should be visible as "heater safety mechanism triggered, please check the temperature sensor and heater!". This should also help preventing jams if the heater gets disconnected.