occasional HTTP 503 errors causes applet to stop updating

[rwx]

Occasionally the app stops updating, and the problem appears that the call to urlopen() is not tolerant of receiving an HTTP error:

Traceback (most recent call last):
  File "/opt/extras.ubuntu.com/my-weather-indicator/share/my-weather-indicator/myweatherindicator.py", line 194, in work
    self.set_menu()
  File "/opt/extras.ubuntu.com/my-weather-indicator/share/my-weather-indicator/myweatherindicator.py", line 462, in set_menu
    weather = self.weatherservice1.get_weather()
  File "/opt/extras.ubuntu.com/my-weather-indicator/share/my-weather-indicator/wundergroundapi.py", line 146, in get_weather
    weather_data = self._get_weather()
  File "/opt/extras.ubuntu.com/my-weather-indicator/share/my-weather-indicator/wundergroundapi.py", line 80, in _get_weather
    json_string = read_from_url(URL%(self.key,self.latitude,self.longitude))
  File "/opt/extras.ubuntu.com/my-weather-indicator/share/my-weather-indicator/comun.py", line 179, in read_from_url
    f = urllib2.urlopen(request)
  File "/usr/lib/python2.7/urllib2.py", line 126, in urlopen
    return _opener.open(url, data, timeout)
  File "/usr/lib/python2.7/urllib2.py", line 406, in open
    response = meth(req, response)
  File "/usr/lib/python2.7/urllib2.py", line 519, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python2.7/urllib2.py", line 444, in error
    return self._call_chain(*args)
  File "/usr/lib/python2.7/urllib2.py", line 378, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.7/urllib2.py", line 527, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 503: Service Unavailable