[Modify] Throw exceptions

websocket-sharp/Server/WebSocketBehavior.cs

@@ -399,7 +399,12 @@ namespace WebSocketSharp.Server
     /// </param>
     protected void Error (string message, Exception exception)
     {
-      if (message != null && message.Length > 0)
+      if (message == null)
+        throw new ArgumentNullException ("message");
+
+      if (message.Length == 0)
+        throw new ArgumentException ("An empty string.", "message");
+
       OnError (new ErrorEventArgs (message, exception));
     }