Adding query string to search form

Fixes #3426 . Adding the query string used in the 404 search to the text field in the search form displayed on the 404 page. 

@aaronschachter @deadlybutter 
@aaronschachter yea, using i'm using `drupal_build_form()` to render this particular search form which allows you to pass in a form state. So it probably doesn't effect other forms rendered on the page. Not seeing errors related to the search for on any other pages, but I added the double check just in case, as I do think it is important to be more defensive.