How to cancel the event in Item Adding and display javascript message and prevent the page from redirecting to the SharePoint Error Page?

The following Event Handler code will prevent users from creating duplicate value in "Title" field.

ItemAdding Event Handler

public override void ItemAdding(SPItemEventProperties properties)
{
base.ItemAdding(properties);
if (properties.ListTitle.Equals("My List"))
{
try
{
using(SPSite thisSite = new SPSite(properties.WebUrl))
{
SPWeb thisWeb = thisSite.OpenWeb();
SPList list = thisWeb.Lists[properties.ListId];
SPQuery query = new SPQuery();
query.Query = @"<Where><Eq><FieldRef Name='Title' /><Value Type='Text'>" + properties.AfterProperties["Title"] + "</Value></Eq></Where>";
SPListItemCollection listItem = list.GetItems(query);
if (listItem.Count > 0)
{
properties.Cancel = true;
properties.ErrorMessage = "Item with this Name already exists. Please create a unique Name.";
}
}
}
catch (Exception ex)
{
PortalLog.LogString("Error occured in event ItemAdding(SPItemEventProperties properties)() @ AAA.BBB.PreventDuplicateItem class. Exception Message:" + ex.Message.ToString());
throw new SPException("An error occured while processing the My List Feature. Please contact your Portal Administrator");
}
}
}

Feature.xml


<?xml version="1.0" encoding="utf-8"?>
<Feature Id="1c2100ca-bad5-41f5-9707-7bf4edc08383"
Title="Prevents Duplicate Item"
Description="Prevents duplicate Name in the "My List" List"
Version="12.0.0.0"
Hidden="FALSE"
Scope="Web"
DefaultResourceFile="core"
xmlns="http://schemas.microsoft.com/sharepoint/">
<ElementManifests>
<ElementManifest Location="elements.xml"/>
</ElementManifests>
</Feature>

Element.xml


<?xml version="1.0" encoding="utf-8" ?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<Receivers ListTemplateId="100">
<Receiver>
<Name>AddingEventHandler</Name>
<Type>ItemAdding</Type>
<SequenceNumber>10000</SequenceNumber>
<Assembly>AAA.BBB, Version=1.0.0.0, Culture=neutral, PublicKeyToken=8003cf0cbff32406</Assembly>
<Class>AAA.BBB.PreventDuplicateItem</Class>
<Data></Data>
<Filter></Filter>
</Receiver>
</Receivers>
</Elements>