Git maintenance fails without meaningful error message if any remote is no longer available

["Anselm Schüler" <mail@anselmschueler.com>]

I have a repo with multiple remotes, one of which no longer exists. When 
git-maintenance runs on it, it fails during the prefetch stage because 
that remote doesn’t exist anymore, and gives a mostly unhelpful error 
message:

$ git maintenance run --schedule=daily
ERROR: Repository not found.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
error: failed to prefetch remotes
error: task 'prefetch' failed

I think that
1. git-maintenance should report which remote it’s encountering an error on
2. git-maintenance should continue fetching other remotes even if one fails

Now, on my system, the systemd timers for git-maintenance use 
git-for-each-repo. Not sure if that’s upstream behaviour or something 
Nix/home-manager does. But if it is upstream behaviour, it would also be 
great to report the repo the error comes from, since I basically had to 
guess right now which repo was erroring. Luckily I have only three repos 
under maintenance so that was fine.

Let me know if you agree that this should be done. I would be open to 
writing a patch (no promises though)

Anselm