Bug 1728662 - Panic if adding fonts fails. r=lsalzman
Bug 1728662 - Panic if adding fonts fails. r=lsalzman

We end up panic later if we try to use a font that failed to be added.

Differential Revision: https://phabricator.services.mozilla.com/D124255

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/8d52d18469fce573961cf7e33a9d9a41a206a2bc

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender