client: use exceptions instead of return states
    
    Python has builtin exceptions which can be used very well to handling
    errors in python instead of returning error states (C style)
    
    Exception will allow better client-server integration in future
    
    https://fedorahosted.org/freeipa/ticket/6392
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>