openshift: return an error if no control plane machine was found

When getting the subnet from machines, let's return an error if no
control plane machine was found. This will make it clearer when
debugging.

Author: EmilienM

openshift/pkg/infraclustercontroller/openstackcluster.go

@@ -355,6 +355,10 @@ func (r *OpenShiftClusterReconciler) getDefaultSubnetFromMachines(ctx context.Co
 		return nil, fmt.Errorf("listing control plane machines: %w", err)
 	}
 
+	if len(mapiMachines.Items) == 0 {
+		return nil, fmt.Errorf("no control plane machines found")
+	}
+
 	apiServerInternalIPs := make([]net.IP, len(platformStatus.APIServerInternalIPs))
 	for i, ipStr := range platformStatus.APIServerInternalIPs {
 		apiServerInternalIPs[i] = net.ParseIP(ipStr)