Add missing awaits for the rebuild_repair and repair background actions.
Although the background actions hold the _async_gate
which is closed in topology_coordinator::run(),
stop() still needs to await all background action futures
and handle any errors they may have left behind.
Fixes#23755
* The issue exists since 6.2
Closesscylladb/scylladb#17712
* github.com:scylladb/scylladb:
topology_coordinator: stop: await all background_action_holder:s
topology_coordinator: stop: improve error messages
topology_coordinator: stop: define stop_background_action helper