Unhandled `git pull` error when tracked branch ref has been deleted on remote

[eighty4]

maestro git has an unhandled error scenario when performing git pull on a repo tracking a delete branch on remote.

1. git checkout -b branch-name
2. git push -u origin branch-name
3. delete branch from remote
4. git pull

Your configuration specifies to merge with the ref 'refs/heads/branch-name'
from the remote, but no such ref was fetched.

Could also be reproduced by manually changing the branch.[branch-name].merge config entry pointing to a ref that does not exist:

1. git clone git@github.com:eighty4/maestro
2. git config branch.main.merge something-bogus