fix(bigtable): error message for LRO (GoogleCloudPlatform#1557)

Author: rsamborski

bigtable/src/create_cluster.php

@@ -92,7 +92,7 @@ function create_cluster(
                 printf('Cluster created: %s', $clusterId);
             } else {
                 $error = $operationResponse->getError();
-                printf('Cluster not created: %s', $error);
+                printf('Cluster not created: %s', $error->getMessage());
             }
         } else {
             throw $e;