When I Enter The Text In Quantity Text Box If The Value Doesn't Exist In Database I Want To Show Error Message

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="SalesOrder.aspx.cs" Inherits="SalesOrder" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">

    <script type="text/javascript" language="javascript">
        function GetTotal() {
            var val1 = document.getElementById('<%=txtq.ClientID%>').value;
            var val2 = document.getElementById('<%=txtp.ClientID%>').value;
            if (val1 != "" && val2 != "") {
                document.getElementById('<%=txttotal.ClientID%>').value = val1 * val2;
            }

        }

    </script>

    <style type="text/css">
        .style9 {
            height: 34px;
        }
    </style>

</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder3" Runat="Server">
    <asp:Panel ID="Panel1" runat="server">
        <br />
        <asp:HyperLink ID="HyperLink12" runat="server" NavigateUrl="~/Customer.aspx">Customer Details</asp:HyperLink>
        <br />
        <br />
        <br />
        <asp:HyperLink ID="HyperLink13" runat="server" NavigateUrl="~/SalesOrder.aspx">Sales Order</asp:HyperLink>
        <br />
        <br />
        <br />
        <br />
        <br />
        <br />
        <br />
        <br />
        <br />
    </asp:Panel>
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="ContentPlaceHolder2" Runat="Server">

    <table>

<tr>
<td align="right">Customer ID</td>
<td>
    <asp:DropDownList ID="ddlcid" runat="server" DataSourceID="SqlDataSource1"
        DataTextField="Customer_Id" DataValueField="Customer_Id" AppendDataBoundItems="true">
        <asp:ListItem>---select Customer id---</asp:ListItem>
    </asp:DropDownList>
    <asp:SqlDataSource ID="SqlDataSource1" runat="server"
        ConnectionString="<%$ ConnectionStrings:sqlserverConnectionString %>"
        SelectCommand="SELECT [Customer_Id] FROM [CustomerTable_]">
    </asp:SqlDataSource>
    </td>
</tr>
<tr>
<td align="right">Product Id</td>
<td>
    <asp:DropDownList ID="ddlpid" runat="server" DataSourceID="SqlDataSource2"
        DataTextField="Productcode" DataValueField="Productcode"
        AppendDataBoundItems="true"
        onselectedindexchanged="ddlpid_SelectedIndexChanged" AutoPostBack="True">
        <asp:ListItem>---select Product Id---</asp:ListItem>
    </asp:DropDownList>
    <asp:SqlDataSource ID="SqlDataSource2" runat="server"
        ConnectionString="<%$ ConnectionStrings:sqlserverConnectionString %>"
        SelectCommand="SELECT [Productcode] FROM [Products]"></asp:SqlDataSource>
    </td>
</tr>
<tr>
<td align="right">Price</td>
<td>
    <asp:TextBox ID="txtp" runat="server"></asp:TextBox>
    </td>
</tr>
<tr>
<td align="right">Quantity</td>
<td>
    <asp:TextBox ID="txtq" runat="server"></asp:TextBox>
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
        ErrorMessage="*" ForeColor="Red" ControlToValidate="txtq"></asp:RequiredFieldValidator>
    </td>
</tr>

<tr>
<td align="right" class="style9">Total</td>
<td class="style9">
    <asp:TextBox ID="txttotal" runat="server"></asp:TextBox>
    <asp:Button ID="add" runat="server" Text="add" OnClientClick="return GetTotal();" />
&nbsp;</td>
</tr>
<tr>
<td align="right">Date Of Delivery</td>
<td>

    <asp:TextBox ID="txtdate" runat="server"></asp:TextBox>
  <asp:CalendarExtender ID="calender1" runat="server" TargetControlID="txtdate"
        Format="dd/MM/yyyy"></asp:CalendarExtender>


    </td>
</tr>
<tr>
<td colspan="2" align="center">

    <asp:Button ID="btnsubmit" runat="server" Text="Submit"
        onclick="btnsubmit_Click" />

</td>
</tr>
</table>

</asp:Content>