git-clean: handle errors if removing files fails

git-clean: handle errors if removing files fails

git-clean simply ignored errors if removing a file or directory failed. This
patch makes it raise a warning and the exit code also greater than zero if
there are remaining files.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>