[oozie] Show error on dashboard when server is down

{
  "message": "An error occurred with Oozie.", 
  "traceback": [
    [
      "/home/romain/projects/hue/apps/oozie/src/oozie/views/dashboard.py", 
      166, 
      "decorate", 
      "return view_func(request, *args, **kwargs)"
    ], 
    [
      "/home/romain/projects/hue/apps/oozie/src/oozie/views/dashboard.py", 
      197, 
      "list_oozie_workflows", 
      "wf_list = oozie_api.get_workflows(**kwargs)"
    ], 
    [
      "/home/romain/projects/hue/desktop/libs/liboozie/src/liboozie/oozie_api.py", 
      127, 
      "get_workflows", 
      "return self.get_jobs('wf', offset, cnt, filters)"
    ], 
    [
      "/home/romain/projects/hue/desktop/libs/liboozie/src/liboozie/oozie_api.py", 
      117, 
      "get_jobs", 
      "resp = self._root.get('jobs', params)"
    ], 
    [
      "/home/romain/projects/hue/desktop/core/src/desktop/lib/rest/resource.py", 
      97, 
      "get", 
      "return self.invoke(\"GET\", relpath, params, headers=headers, allow_redirects=True)"
    ], 
    [
      "/home/romain/projects/hue/desktop/core/src/desktop/lib/rest/resource.py", 
      78, 
      "invoke", 
      "urlencode=self._urlencode)"
    ], 
    [
      "/home/romain/projects/hue/desktop/core/src/desktop/lib/rest/http_client.py", 
      161, 
      "execute", 
      "raise self._exc_class(ex)"
    ]
  ], 
  "detail": "('Connection aborted.', error(111, 'Connection refused'))", 
  "title": "Error"
}