Oops, that’s a step I forgot to describe. It’s basically changing the Updater.h in esp8266 core to include an additional function (to handle firmware upload failure case). For now, check the opengarage github:
https://github.com/OpenGarage/OpenGarage-Firmware/tree/master/Modifications
which contains the Updater.h file that should overwrite the existing one in the esp8266 core folder (or if it’s not compatible with your Updater.h, just follow the changes in that file to add the additional function).