Catch unsigned logout requests and raise a 400 for now
    
    A 400 is still going to blow up the logout sequence but
    it is better than a 500 and at least tells the user what
    is wrong.
    
    This is most likely to be run into during initial SP
    testing and not in production.
    
    https://fedorahosted.org/ipsilon/ticket/166
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>