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 
It might just be part of the way a drupal `search_form` is rendered.  If errors aren't not showing up in places like the homepage and stuff, feel free to keep as is.