[Discovery] Improved logging when a join request is not executed because local node is no longer master

src/main/java/org/elasticsearch/discovery/zen/ZenDiscovery.java

@@ -797,7 +797,11 @@ public class ZenDiscovery extends AbstractLifecycleComponent<Discovery> implemen
 
                 @Override
                 public void onFailure(String source, Throwable t) {
+                    if (t instanceof ClusterService.NoLongerMasterException) {
+                        logger.debug("not processing [{}] as we are no longer master", source);
+                    } else {
                         logger.error("unexpected failure during [{}]", t, source);
+                    }
                     callback.onFailure(t);
                 }