Today, when the `Fixes` prefix is missing or the developer is not a collaborator with `scylladbbot` we remove the backport labels to prevent the process from starting and notifying the developers.
Developers are worried that removing these backport labels will cause us to forget we need to do these backports. @nyh suggested to add a `scylladbbot/backport_error` label instead
Applied those changes, so when a `Fixes` prefix is missing we will add a `scylladbbot/backport_error` label and stop the process
When a user doesn't accept the invite we will still open the PR but he will not be assigned and will not be able to edit the branch when we have conflicts
Fixes: https://github.com/scylladb/scylla-pkg/issues/4898
Fixes: https://github.com/scylladb/scylla-pkg/issues/4897Closesscylladb/scylladb#23259