Nodefs errno values don't match emscripten errno values on Windows

e.g. on our current Windows bot, in `test_unistd_access`, `test_unistd_misc`, and `test_unistd_unlink`, access to the "nonexistent" file returns `ENOENT` which is 2 on posix and 4058 on Windows. NODEFS seems to pass the underlying errno value through from Node, which presumably passes it through from the platform.

I assume this is a real problem since we use posix errno values in emscripten. Has this ever worked, or did it used to work in a different way?