Issue #21775: shutil.copytree(): fix crash when copying to VFAT

Issue #21775: shutil.copytree(): fix crash when copying to VFAT

An exception handler assumed that that OSError objects always have a
'winerror' attribute. That is not the case, so the exception handler
itself raised AttributeError when run on Linux (and, presumably, any
other non-Windows OS).

Patch by Greg Ward.