Handle the connection failure exception in weather segment to avoid glitching

When the network goes down, weather update will fail to resolve the host and thus `urllib2` will throw an `URLError` exception:

    2014-05-30 04:40:56,619:ERROR:tmux:WeatherSegment:Exception while updating: <urlopen error [Errno 111] Connection refused>
    Traceback (most recent call last):
      File "/home/ymf/.local/lib64/python2.7/site-packages/powerline/lib/threaded.py", line 68, in set_update_value
        self.update_value = self.update(self.update_value)
      File "/home/ymf/.local/lib64/python2.7/site-packages/powerline/segments/common.py", line 380, in update
        raw_response = urllib_read(self.url)
      File "/home/ymf/.local/lib64/python2.7/site-packages/powerline/lib/url.py", line 14, in urllib_read
        return urlopen(url, timeout=10).read().decode('utf-8')
      File "/usr/lib64/python2.7/urllib2.py", line 127, in urlopen
        return _opener.open(url, data, timeout)
      File "/usr/lib64/python2.7/urllib2.py", line 404, in open
        response = self._open(req, data)
      File "/usr/lib64/python2.7/urllib2.py", line 422, in _open
        '_open', req)
      File "/usr/lib64/python2.7/urllib2.py", line 382, in _call_chain
        result = func(*args)
      File "/usr/lib64/python2.7/urllib2.py", line 1214, in http_open
        return self.do_open(httplib.HTTPConnection, req)
      File "/usr/lib64/python2.7/urllib2.py", line 1184, in do_open
        raise URLError(err)
    URLError: <urlopen error [Errno 111] Connection refused>