submodule--helper: libify "must_die_on_failure" code paths

submodule--helper: libify "must_die_on_failure" code paths

In preceding commits the codepaths around update_submodules() were
changed from using exit() or die() to ferrying up a
"must_die_on_failure" in the cases where we'd exit(), and in most
cases where we'd die().

We needed to do this this to ensure that we'd early exit or otherwise
abort the update_submodules() processing before it was completed.

Now that those preceding changes have shown that we've converted those
paths, we can remove the remaining "ret == 128" special-cases, leaving
the only such special-case in update_submodules(). I.e. we now know
after having gone through the various codepaths that we were only
returning 128 if we meant to early abort.

In update_submodules() we'll for now set any non-zero non-128 exit
codes to "1", but will start ferrying up the exit code as-is in a
subsequent commit.

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>