[XeTeX] Checking if a font exists

[Akira Kakuto]

> > * The error code is set because of the error message, if the
> >   font cannot be found.
> 
>   Doesn't XeTeX implement \suppressfontnotfounderror that was added to
> pdfTeX two or three years ago, precisely to deal with this kind of
> problem?

The primitive exists in 1.50.0-alpha-20080414 in the trunk.
However 1.50.0 is not maintained recently.
For example, 1.50.0 does not merge the new synctex.

Best,
Akira