Font: attempt fallback font, abort if no fonts found.

Font: attempt fallback font, abort if no fonts found.

If you happen to have a font_path setting that is incorrect,
minetest will just attempt to start the gui without a valid
font which leads to a segfault later on.

We can attempt to load the fallback font path fairly easy,
but if that fails we should give up with a proper error message
and not a weird segfault later. This forces an abort() if
the fallback fails as well, and prints a useful error
message to the console.