tolerance for invalid lookups

I’m not sure what causes these lookups but I remember that they were present from the start. Only that we worked around them for non-MS systems early, while the problem remained undetected on the Windows side. It is not advisable to make the request parser more tolerant (like the patch I posted earlier on lualatex-dev) as this would mean giving up font features in ``anon:`` lookups. So we test with each lookup if the original specification is a loadable file -- crude, but effective, and very unlikely to match the wrong font.

As to the cause of these lookups, loading fontspec clearly triggers them, and they surface when fonts from the EU2 encoding are requested.