git-submodule.sh: clarify the "should we die now" logic

git-submodule.sh: clarify the "should we die now" logic

Earlier the decision to stop or continue was made on the $action variable
that was set by inspecting $update_module variable. The former is a
redundant variable and will be removed in another topic.

Decide upon inspecting $update_module if a failure should cascade up to
cause us immediately stop, and use a variable that means just that, to
clarify the logic.

Incidentally this also makes the merge with the other topic slightly
easier and cleaner to understand.

Signed-off-by: Junio C Hamano <gitster@pobox.com>