Hi Shelwin,
The error reporting in Web Dynpro ABAP is done using message areas. You need to insert a message area UI element into your view.
Following link contains step by step information on how to create a message area UI element and trigger custom messages.
http://webdynproabap.wordpress.com/2012/07/08/message-handling/
If you need anymore specifics please let me know.
Regards,
Karan Ranawat