[font-loading] How should invalid font resources fail?

[RByers]

The load algorithm is a little vague on whether or not it's an error to successfully load a resource which is not a valid font (and I don't see any tests in csswg-test for this spec to help clarify the expectation here). I assume we wouldn't want to have a FontFace instance for something that doesn't represent a valid font, and so it should be an error - but the spec should probably call that out explicitly.

This come up in the review of this web-platform-test change.