Don't assume all connection managers support Sugar

If the user has a "random" telepathy connection manager
installed and running on the system (eg. telepathy idle being
active if they are chatting on irc with GNOME's Polari), activities
will crash on startup.

Steps to reproduce the crash:
1. Run Polari (eg. via the Terminal activity)
2. Join an irc channel in Polari (eg. #sugar on freenode)
3. Try to open an activity from the home view
    - notice that opening any sugar3 activity will crash

src/sugar3/presence/presenceservice.py

@@ -91,6 +91,11 @@ class PresenceService(GObject.GObject):
                     if e.get_dbus_name() == name:
                         logging.debug("There's no shared activity with the id "
                                       "%s" % activity_id)
+                    elif e.get_dbus_name() == \
+                         'org.freedesktop.DBus.Error.UnknownMethod':
+                        logging.warning(
+                            'Telepathy Account %r does not support '
+                            'Sugar collaboration', account_path)
                     else:
                         raise
                 else: