Google now offer a widget for customized 404 pages.

The code is below

<script type="text/javascript">
  var GOOG_FIXURL_LANG = 'en';
  var GOOG_FIXURL_SITE = 'http://mobizong.com/';
</script>
<script type="text/javascript"
    src="http://linkhelp.clients.google.com/tbproxy/lh/wm/fixurl.js"></script>

Here in this code you can replace mobizong.com with your own site url.
The output of this script is like.

Example

If a user types in the incorrect url http://www.example.com/news/recentz we’ll display the following: