submodule--helper: don't exit() on failure, return

submodule--helper: don't exit() on failure, return

Change code downstream of module_update() to short-circuit and return
to the top-level on failure, rather than calling exit().

To do so we need to diligently check whether we "must_die_on_failure",
which is a pattern started in c51f8f94e5b (submodule--helper: run
update procedures from C, 2021-08-24), but which hadn't been completed
to the point where we could avoid calling exit() here.

This introduces no functional changes, but makes it easier to both
call these routines as a library in the future, and to eventually
avoid leaking memory.

This and similar control flow in submodule--helper.c could be made
simpler by properly "libifying" it, i.e. to have it consistently
return -1 on failures, and to early return on any non-success.

But let's leave that larger project for now, and (mostly) emulate what
were doing with the "exit(128)" before this change.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>