src/dired.c (directory_files_internal, file_name_completion)
[WINDOWSNT]: Signal an error when errno is set non-zero by
'readdir', regardless of its value.
src/w32.c (sys_readdir): Set errno to ENOENT when the directory
doesn't exist and to EACCES when it's not accessible to the
current user. Set errno to zero when FindNextFile exhausts the
directory, so that callers don't interpret that as an error and
don't signal a file-error.
(open_unc_volume): Set errno to ENOENT if WNetOpenEnum fails.