Replace exit() calls with exceptions
    
    In order to enable correct handling of hard errors from within the
    composite installer code, all calls to `sys.exit` have to be replaced
    with raising ScriptError.
    
    https://fedorahosted.org/freeipa/ticket/6629
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>