[sugar] Presence service bug

[Marco Pesenti Gritti]

Hi Dan,

I get this when trying to unregister a service...

Traceback (most recent call last):
  File 
"/home/marco/sugar-jhbuild/source/sugar/shell/PresenceService/PresenceService.py", 
line 528, in _service_disappeared_cb
    del self._services[key]
KeyError: (u'marco [a27cf19c9d744c9cba3eef9a9d1f0cea24b56a87]', 
u'_web_olpc._udp')

If you could have a look that would be helpful :)
To reproduce:

- Open an activity
- Share it
- Close (Alt+c)

Marco