mingw: be more informative when ownership check fails on FAT32

mingw: be more informative when ownership check fails on FAT32

The FAT file system has no concept of ACLs. Therefore, it cannot store
any ownership information anyway, and the `GetNamedSecurityInfoW()` call
pretends that everything is owned "by the world".

Let's special-case that scenario and tell the user what's going on.

This addresses https://github.com/git-for-windows/git/issues/3886

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>