Cancel if bus not available in client

src/CommandBusClient.php

@@ -144,7 +144,7 @@ class CommandBusClient implements CommandBusInterface
     {
         if (!$this->connection) {
             return new Promise(function (callable $resolve, callable $canceller) {
-                $canceller("Not connected to command bus.");
+                $canceller(new \RuntimeException("Not connected to command bus."));
             });
         }