builtin/rm.c: use warning_errno()

While at there, improve the message a bit (what operation failed?) and
mark it for translation since the format string is now a sentence.

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

builtin/rm.c

@@ -152,7 +152,7 @@ static int check_local_mod(unsigned char *head, int index_only)
 
 		if (lstat(ce->name, &st) < 0) {
 			if (errno != ENOENT && errno != ENOTDIR)
-				warning("'%s': %s", ce->name, strerror(errno));
+				warning_errno(_("failed to stat '%s'"), ce->name);
 			/* It already vanished from the working tree */
 			continue;
 		}