I always make sure my battery level is fully charged before flashing. Would be even better if safestrap checks battery level before flashing or at least warns you.

@TheKit I don't care that much for stock OS so will replace that for SFOS or Lineage OS anyway. Any chance you will make it possible to flash stock with SFOS later?

@QWERTY_Slider pretty sure you are not the first with an issue like this and is not SFOS specific. You can also post your problem at the XDA forum if TheKit's suggestion doesn't work.
https://forum.xda-developers.com/droid-4