Added user friendly error message for dnszone enable and disable
    
    Added try-except block in dns plugin in order to provide user
    friendly message to end user.
    
    https://fedorahosted.org/freeipa/ticket/4811
    
    Signed-off-by: Abhijeet Kasurde <akasurde@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>