como posso capturar os erros 404 e 403 para páginas em python e urllib (2), por exemplo?

Existem maneiras rápidas sem grandes invólucros de classe?

Informações adicionadas (rastreamento de pilha):

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    page = urllib2.urlopen("http://localhost:4444")
  File "/usr/lib/python2.6/urllib2.py", line 126, in urlopen
    return _opener.open(url, data, timeout)
  File "/usr/lib/python2.6/urllib2.py", line 391, in open
    response = self._open(req, data)
  File "/usr/lib/python2.6/urllib2.py", line 409, in _open
    '_open', req)
  File "/usr/lib/python2.6/urllib2.py", line 369, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.6/urllib2.py", line 1161, in http_open
    return self.do_open(httplib.HTTPConnection, req)
  File "/usr/lib/python2.6/urllib2.py", line 1136, in do_open
    raise URLError(err)
urllib2.URLError: <urlopen error [Errno 111] Connection refused>