detect mDNS issues and report meaningful message back to the user

[Antoine Martin]

ie, on a system where avahi was not started? (or was the port blocked?), this error occurred (which is obscure to say the least):

Exception in thread avahi_publisher for: [('0.0.0.0', 35941)]:
Traceback (most recent call last):
  File "/usr/lib64/python2.6/threading.py", line 525, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/site-packages/shifter/net/avahi_publisher.py", line 138, in run
    self.bus.get_object(avahi.DBUS_NAME, avahi.DBUS_PATH_SERVER),
  File "/usr/lib/python2.6/site-packages/dbus/bus.py", line 244, in get_object
    follow_name_owner_changes=follow_name_owner_changes)
  File "/usr/lib/python2.6/site-packages/dbus/proxies.py", line 241, in __init__
    self._named_service = conn.activate_name_owner(bus_name)
  File "/usr/lib/python2.6/site-packages/dbus/bus.py", line 178, in activate_name_owner
    return self.get_name_owner(bus_name)
  File "/usr/lib/python2.6/site-packages/dbus/bus.py", line 355, in get_name_owner
    's', (bus_name,), utf8_strings=True)
  File "/usr/lib/python2.6/site-packages/dbus/connection.py", line 630, in call_blocking
    message, timeout)
DBusException: org.freedesktop.DBus.Error.NoMemory: Not enough memory