When setting the bed temp via M140/M190 if the thermistor does not read an increase of WATCH_BED_TEMP_INCREASE degrees by WATCH_BED_TEMP_PERIOD seconds then it will throw "Error:Heating failed, system stopped! Heater_ID: bed" and call the kill() function.
Conflicts:
Marlin/Configuration_adv.h
Since residency_start_ms is -1 when entering the heatup while loop whilst the hotend temp is close to the target them already then it may not escape.
Hence "Start the TEMP_RESIDENCY_TIME timer when we reach target temp for the first time."