When any ASP.Net RequiredFieldValidator validation fails display generic error message to user

Check this sample

    <div id = "message" style = "display:none">Please fill all the fields.</div>
    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate = "TextBox1" ErrorMessage="Required"></asp:RequiredFieldValidator>
    <asp:Button ID="btnSubmit" runat="server" Text="Submit" />
    <script type="text/javascript">
        function WebForm_OnSubmit() {
            var message = document.getElementById("message");
            message.style.display = "none";
            if (typeof (ValidatorOnSubmit) == "function" && ValidatorOnSubmit() == false) {
                for (var i in Page_Validators) {
                    try {
                        if (!Page_Validators[i].isvalid) {
                            message.style.display = "block";
                        }
                    } catch (e) {
                    }
                } return false;
            } return true;
        }
    </script>

Refer

Display ASP.Net ValidationSummary control inside jQuery Modal Dialog Popup