g_file_make_directory_with_parents: Fix error propagation

When creating a directory fails for some reason other than
the parent not existing, don't clear the error before we try
to propagate it.

To reproduce, run 'ostadmin init' on /ostree or otherwise try to
run the function on a directory with a parent directory where the
current user is not allowed to write.

https://bugzilla.gnome.org/show_bug.cgi?id=680823