1612339 - Improve error handling and reporting in FontFace

Status: NEW

(Core :: Layout, defect, P3)

Description

[Boris Zbarsky [:bzbarsky]]

There are a few issues:

1. FontFace::EnsurePromise can fail to create mLoaded but then still dereference it. It should instead take an ErrorResult arg that it passes through to Promise::Create and immediately bail out if it fails.
2. Errors are propagated through as nsresult values. It might be better to propagate them through as ErrorResults so that we can provide a useful message describing the error at the point when we detect that we have an error. We might need something else if we might need to reject multiple promises with the same error?