MediaClient: Fix -Werror=set-but-unused

* The error wasn't checked.

src/kits/media/experimental/MediaClient.cpp

@@ -337,6 +337,9 @@ BMediaClient::Start()
 				remoteNode, fNode->TimeSource()->Now());
 	}
 
+	if (err != B_OK)
+		return err;
+
 	return BMediaRoster::CurrentRoster()->StartNode(
 		fNode->Node(), fNode->TimeSource()->Now());
 }