AWS: If we have no subnets, bail out early

We know the ELB call will fail, so we error out early rather than
hitting the API.  Preserves rate limit quota, and also allows us to give
a more self-evident message.

Fix #21993

pkg/cloudprovider/providers/aws/aws.go

@@ -2149,6 +2149,11 @@ func (s *AWSCloud) EnsureLoadBalancer(name, region string, publicIP net.IP, port
 		return nil, err
 	}
 
+	// Bail out early if there are no subnets
+	if len(subnetIDs) == 0 {
+		return nil, fmt.Errorf("could not find any suitable subnets for creating the ELB")
+	}
+
 	// Create a security group for the load balancer
 	var securityGroupID string
 	{