Bug#900777: fontmake: fails to rebuild fonts-firacode from its glyphs source

[James Godfrey-Kittle]

It's possible that this font actually contains illegal feature syntax,
which is still accepted by Glyphs. Or of course it could be an issue
with some component of fontmake; it's worth posting an issue on Github
to have it triaged (I was planning to do this soon when I have time).