fix: Make error message more clear about where ordering must be enabled when publishing (#293)

* feat: Add flow control support to publisher

* make suggested fixes

* chore: Remove note that ordering keys requires enablements.

* feat: Add support for server-side flow control

* Revert "chore: Remove note that ordering keys requires enablements."

This reverts commit 9c113c3.

* fix: Fix import order

* fix: Make error message more clear about where ordering must be enabled when publishing.

Author: kamalaboulhosn

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Publisher.java

@@ -233,7 +233,9 @@ public ApiFuture<String> publish(PubsubMessage message) {
     final String orderingKey = message.getOrderingKey();
     Preconditions.checkState(
         orderingKey.isEmpty() || enableMessageOrdering,
-        "Cannot publish a message with an ordering key when message ordering is not enabled.");
+        "Cannot publish a message with an ordering key when message ordering is not enabled in the "
+            + "Publisher client. Please create a Publisher client with "
+            + "setEnableMessageOrdering(true) in the builder.");
 
     final OutstandingPublish outstandingPublish =
         new OutstandingPublish(messageTransform.apply(message));