fix: Make change stream unknown mod error more actionable (#1938)

* fix: Make change stream unknown mod error more actionable

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Igor Bernstein <igorbernstein@google.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/changestream/ChangeStreamStateMachine.java

@@ -486,7 +486,8 @@ State handleDataChange(ReadChangeStreamResponse.DataChange dataChange) {
                   Value.fromProto(mod.getAddToCell().getTimestamp()),
                   Value.fromProto(mod.getAddToCell().getInput()));
             }
-            throw new IllegalStateException("AWAITING_NEW_DATA_CHANGE: Unexpected mod type");
+            throw new IllegalStateException(
+                "Received unknown mod type. You may need to upgrade your Bigtable client.");
           }
 
           // After adding all mods from this DataChange to the state machine, finish the current