ASP.NET file upload

I have web form where I need to upload a file. I have set the maximum file size size as 10mb in web.config file. Now I need to validate the file upload in the form. If I try to upload a file size greater then 10mb it is moving toHTTP Error 404.13 - Not Found page but instead I need to display a error message in the same form as The maximum file has been extended . I used the following code for that purpose.
 

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 protected void btnSave_OnClick(object sender, EventArgs e)
    {
        if (uxFileUpload.FileBytes.Length > 0 )
        {
            int iMaxFileSize = 2097152;
            if (Request.ContentLength > iMaxFileSize)
            {
                exceedErrormessage.Visible=true; 
                //Server.Transfer("Error.aspx"); 
            }

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<divclass="file-upload"><asp:LabelID="uiFileUpload"runat="server"AssociatedControlID="uxFileUpload"Text="Select file"/><p>The maximun file size is 20mb</p><asp:FileUploadID="uxFileUpload"runat="server"/><asp:PlaceHolderID="exceedErrormessage"Visible="false"runat="server"><asp:LiteralID="exServerErroemessage"runat="server"Text="The maximum file limit has been exceeded"/></asp:PlaceHolder><%--<asp:Label ID="label" runat="server"></asp:Label>--%><asp:CustomValidatorID="CustomValidator1"runat="server"ControlToValidate="uxFileUpload"ErrorMessage="File size should not be greater than 20 mb."OnServerValidate="btnSave_OnClick"></asp:CustomValidator><asp:ButtonID="btnSaveDocument"ValidationGroup="SaveFile"OnClick="btnSave_OnClick"runat="server"Text="Upload"CssClass="button"/></div>

 
But still it is moving to the same error page when I try to upload a file size greater then 10mb. I even tried placing a breakpoint and checked where the problem is occurring. I found that when I try to upload a file size greater than 10 mb it is not even moving to the btnSave_OnClick function but otherwise if I upload a file size less than 10 mb it is moving to the function and the corresponding operation is been performed. Can anyone help me to solve this problem.