Just spotted this project on Twitter.....
https://www.kickstarter.com/projects/resumer3d/resumer-3d-resume-your-interrupted-3d-prints-autom/description
It's allegedly a plug and play module for your printer that acts as a failsafe for power outages and also allows you to resume interrupted prints.
The most interesting thing is that it's less than $30 which makes it probably worth a punt.