Improve error message when track repository is not present on remote machine

Source: https://discuss.elastic.co/t/must-be-a-git-repository/143773

When `--load-driver-hosts` is used, the track repository must be checked out on every host. If this is not possible (no remote configured), Rally raises the error `/path/to/repo must be a git repository` but provides no further guidance to the user. It would be good if we could detect this specific situation and provide a more helpful error message.