I have downloaded FontAwesome using npm and then copied the css-file and the fonts into the right folders in the root-diretory of my electron-application using grunts copy t
I'm sure this is solved, but this worked for me, so... I'm gonna leave this here:
I just had the same issue with a font I had used before. Turned out this was caused by a problem with FTP. The file was uploaded as text (ASCII) instead of binary, which corrupted the file. I simply re-uploaded the font files, and then it all worked.