Block the launch of Grafana if alerting migration happens and somehow fails

[armandgrillet]

This will only happen for users migrating from legacy alerting to Grafana Alerting when upgrading from Grafana < 11 to Grafana >= 11.0.

If this happens, we want them to receive an error telling them to use 10.4 so that they can use dry-run and do the migration. In Grafana >= 11.0, the legacy alerting codebase will not exist anymore thus the dry-run will also be gone.