transport-helper: demote lack of "force" option to a warning

It would have been a good conservative position to take, if there
were no existing helpers, to die when the user asked to force a push
through a transport helper mechanism and the helper script hasn't
been updated to handle the "force" option.

There however are existing helpers in the field and none of them
obviously has been taught about the option yet.  If a helper has not
been updated to understand "force", but the user has happily been
accepting the result of "git push $there +master" (perhaps because
he has never pushed a non-ff history so far), the change made
previously in this series will fail the push, which would be a minor
regression for such a user.

Demote lack of "force" option from a "die()" to a "warning()".  By
doing so, such a user now gets a warning that notifies him that his
push has been working by accident, even when the particular ff-push
that showed the error message goes through successfully.

If his push does need "force", he gets the same old behaviour,
possibly a broken one, out of the helper that has not been updated,
but again he does see the warning.

Helped-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>