[Modify] Throw exception

websocket-sharp/Server/WebSocketSessionManager.cs

@@ -979,7 +979,11 @@ namespace WebSocketSharp.Server
     public void SendTo (byte[] data, string id)
     {
       IWebSocketSession session;
-      if (TryGetSession (id, out session))
+      if (!TryGetSession (id, out session)) {
+        var msg = "The session could not be found.";
+        throw new ArgumentException (msg, "id");
+      }
+
       session.Context.WebSocket.Send (data);
     }