You can trap errors using the the CallbackError client event. Add the client event and define a handler- if you want one that displays nothing at all, for example, you would just do:

  <script>
        function Callback_Error(sender, args) {} //trap the error; do nothing. 
    </script>

<ComponentArt:Callback runat="server" ID="Callback1" OnCallback="Callback1_Callback">
            <Content>
                <asp:Label runat="server" ID="lbl1" Text="default"></asp:Label>
            </Content>
            <ClientEvents>
                <CallbackError EventHandler="Callback_Error" />
            </ClientEvents>
        </ComponentArt:Callback>