dir.c: stop ignoring opendir() error in open_cached_dir()

dir.c: stop ignoring opendir() error in open_cached_dir()

A follow-up to the recently fixed bugs in the untracked
invalidation. If opendir() fails it should show a warning, perhaps
this should die, but if this ever happens the error is probably
recoverable for the user, and dying would just make things worse.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>