Update received same error running the bash script:
curl -sL https://raw.githubusercontent.com/MichMich/MagicMirror/master/installers/raspberry.sh | bash

on Raspberry pi 3.0 -B. Installed Raspbian Jesse using NOOBS.

Ran the script again and the response told me NodeJs was installed, “Magic mirror is already installed”
I believe this is more an unintended response of the actual bug. NodeJS was already installed correctly?

Anyway,

navigated into the folder MagicMirror:
npm start -> electron not installed fail.
npm install -> npm start

Bingo, we are running. No need to download any packages seperately.