Check if subnets are empty before passing to the instance provider

pkg/cloudprovider/aws/subnets.go

@@ -57,6 +57,11 @@ func (s *SubnetProvider) Get(ctx context.Context, provisioner *v1alpha1.Provisio
 	if len(constraints.Zones) != 0 {
 		subnets = filterSubnets(subnets, withSubnetZone(predicates.WithinStrings(constraints.Zones)))
 	}
+	// 4. Fail if no subnets found, since the constraints may be violated and
+	// node cannot connect to the API Server.
+	if len(subnets) == 0 {
+		return nil, fmt.Errorf("no subnets exist given constraints")
+	}
 	return subnets, nil
 }