branch: die on config error when unsetting upstream

branch: die on config error when unsetting upstream

When we try to unset upstream configurations we do not check
return codes for the `git_config_set` functions. As those may
indicate that we were unable to unset the respective
configuration we may exit successfully without any error message
while in fact the upstream configuration was not unset.

Fix this by dying with an error message when we cannot unset the
configuration.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>