[XeTeX] Overfull boxes return status of 0 in XeTeX

[David Carlisle]

>  non-zero status codes should be reserved for various categories
> of warning, error, severe error, fatal error, etc., should they not ?

No I think the (now, whatever the convention in vms was) normal convention
is that programs have a clear distinction between warnings and errors.

The engine, and the user code in the document itself should be free to
give warnings safe in the knowledge that they _won't_ stop a build with
make etc.

David