When there are errors starting the first cluster(s) the logs of the server logs are needed. So move `.start()` to the `try` block in `test.py` (out of `asynccontextmanager`).
While there, make `ScyllaClusterManager.start()` idempotent.
Closes#11594
* github.com:scylladb/scylladb:
test.py: fix ScyllaClusterManager start/stop
test.py: fix topology init error handling