When we add or set new branches (e.g. by `git remote add -f` or
`git remote set-branches`) we do not check for error codes when
writing the branches to the configuration file. When persisting
the configuration failed we are left with a remote that has none
or not all of the branches that should have been set without
notifying the user.
Fix this issue by dying early on configuration error.
Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>