does not retry temporary failures

lazr.restfulclient when used on LP will frequently explode due to HTTP 502 or 503 errors that could be avoided if a given request were retried. This should be handled in the restfulclient code so that callers do not have to catch and retry on 502/503. E.g.:

Traceback (most recent call last):
  File "./d2u.py", line 332, in <module>
    if __name__ == '__main__': main()
  File "./d2u.py", line 238, in main
    if source.distro_series.active:
  File "/usr/lib/pymodules/python2.6/lazr/restfulclient/resource.py", line 587,
in __getattr__
    return super(Entry, self).__getattr__(name)
  File "/usr/lib/pymodules/python2.6/lazr/restfulclient/resource.py", line 302,
in __getattr__
    return self.lp_get_parameter(attr)
  File "/usr/lib/pymodules/python2.6/lazr/restfulclient/resource.py", line 193,
in lp_get_parameter
    self._ensure_representation()
  File "/usr/lib/pymodules/python2.6/lazr/restfulclient/resource.py", line 332,
in _ensure_representation
    representation = self._root._browser.get(self._wadl_resource)
  File "/usr/lib/pymodules/python2.6/lazr/restfulclient/_browser.py", line 227,
in get
    response, content = self._request(url, extra_headers=headers)
  File "/usr/lib/pymodules/python2.6/lazr/restfulclient/_browser.py", line 217,
in _request
    raise HTTPError(response, content)
lazr.restfulclient.errors.HTTPError: HTTP Error 503: Service Unavailable