Fix the error handling in client.InitProducerID()

err was shadowed in InitProducerID() so it would never return a non-nil
error. It now returns ErrOutOfBrokers if there aren't any registered brokers.
If there are registered brokers and the broker calls fail then it returns
the last encoutered error.

This was causing a panic in newTransactionManager() with idempotent
production enabled because nil, nil was being returned if the brokers were
unavailable.

client.go

@@ -197,11 +197,12 @@ func (client *client) Brokers() []*Broker {
 }
 
 func (client *client) InitProducerID() (*InitProducerIDResponse, error) {
-	var err error
+	err := ErrOutOfBrokers
 	for broker := client.any(); broker != nil; broker = client.any() {
+		var response *InitProducerIDResponse
 		req := &InitProducerIDRequest{}
 
-		response, err := broker.InitProducerID(req)
+		response, err = broker.InitProducerID(req)
 		switch err.(type) {
 		case nil:
 			return response, nil
@@ -212,6 +213,7 @@ func (client *client) InitProducerID() (*InitProducerIDResponse, error) {
 			client.deregisterBroker(broker)
 		}
 	}
+
 	return nil, err
 }