Do not let pagure return 500 when hit with bogus URL
    
    This should among other stop having pagure return 500 errors and email the
    admins when the URL accessed is, for example:
       https://pagure.io/apple-touch-icon-152x152-precomposed.png
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>