exception handling

Question

i have created a downloader page which downloads the file whenever we fill the textbox with file name and then clicked the download button to download

but when we put the wrong file name in textbox and then clicked the download button then it redirect me to the error page by default but i want to show a message that file not found instead of redirecting to a error page.

 

i want to show a message that file is not found instead of going the error page