off the top of my head possibilities
I can think of 2 options for this.
you could consider a wildcard in the search, so that even if the url was mistyped, it would still find a result, ie department like %lighti%
alternatively' you could consider a conditional php redirect, so that if the search returns no results, redirect to another page (noresults.php)
you could also pass the url parameter to the noresults page and use this in the content and as a more general search parameter, with a message like, 'you typed this', there is nothing, these are the closest match..., with links back to the relevant page.