Add thermal runaway safeguards

[hallboyone]

There is currently nothing checking that the boiler is responding as expected to the heater inputs. This can lead to thermal runaways should the sensor come detached or fail. Functionality should be added to monitor the state of the boiler and enter an error state should a thermal runaway be sensed.
This could be modeled after the thermal runaway logic in the Marlin 3D printer firmware. The basic rules are

1. The temp must be changing at some minimum rate when the machine is settling to a temperature (i.e. responding to control inputs)
2. The temp must not deviate too far from the setpoint after settling (i.e. sensor fails as in CRITICAL - Occasional erratic behavior in boiler #16)