safe_create_leading_directories(): set errno on SCLD_EXISTS

safe_create_leading_directories(): set errno on SCLD_EXISTS

The exit path for SCLD_EXISTS wasn't setting errno, which some callers
use to generate error messages for the user. Fix the problem and
document that the function sets errno correctly to help avoid similar
regressions in the future.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>