Rusttype doesn't return errors

Created by: jimblandy

It would be nice if rusttype returned errors when it encountered data it can't handle, instead of simply returning Option. For example, FontCollection::into_font offers no way to distinguish between failure because the collection contains only one font and failure because the font data is ill-formed.

It seems like it would be idiomatic for Rusttype to have its own Error and Result types. FontCollection::into_font would return Result<Font<'a>>; IntoFontsIter<'a>::Item would be Result<Font<'a>>; and so on.

If the rusttype developers agree that this would be an improvement, I could work on this.