exit gracefully when docker socket is not available

[TomasTomecek]

  File "/usr/lib/python3.4/site-packages/requests/packages/urllib3/connectionpool.py", line 353, in _make_request
    conn.request(method, url, **httplib_request_kw)
  File "/usr/lib64/python3.4/http/client.py", line 1090, in request
    self._send_request(method, url, body, headers)
  File "/usr/lib64/python3.4/http/client.py", line 1128, in _send_request
    self.endheaders(body)
  File "/usr/lib64/python3.4/http/client.py", line 1086, in endheaders
    self._send_output(message_body)
  File "/usr/lib64/python3.4/http/client.py", line 924, in _send_output
    self.send(msg)
  File "/usr/lib64/python3.4/http/client.py", line 859, in send
    self.connect()
  File "/usr/lib/python3.4/site-packages/docker/unixconn/unixconn.py", line 41, in connect
    sock.connect(self.unix_socket)
requests.packages.urllib3.exceptions.ProtocolError: ('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.4/site-packages/docker/client.py", line 89, in _retrieve_server_version
    return self.version(api_version=False)["ApiVersion"]
  File "/usr/lib/python3.4/site-packages/docker/api/daemon.py", line 78, in version
    return self._result(self._get(url), json=True)
  File "/usr/lib/python3.4/site-packages/docker/client.py", line 110, in _get
    return self.get(url, **self._set_request_timeout(kwargs))
  File "/usr/lib/python3.4/site-packages/requests/sessions.py", line 480, in get
    return self.request('GET', url, **kwargs)
  File "/usr/lib/python3.4/site-packages/requests/sessions.py", line 468, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/lib/python3.4/site-packages/requests/sessions.py", line 576, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python3.4/site-packages/requests/adapters.py", line 412, in send
    raise ConnectionError(err, request=request)
requests.exceptions.ConnectionError: ('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib64/python3.4/threading.py", line 921, in _bootstrap_inner
    self.run()
  File "/usr/lib64/python3.4/threading.py", line 869, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.4/site-packages/sen/tui/widget.py", line 430, in realtime_updates
    for content in self.d.realtime_updates():
  File "/usr/lib/python3.4/site-packages/sen/docker_backend.py", line 441, in realtime_updates
    for event in self.client.events(decode=True):
  File "/usr/lib/python3.4/site-packages/sen/docker_backend.py", line 413, in client
    self._client = docker.AutoVersionClient()
  File "/usr/lib/python3.4/site-packages/docker/client.py", line 325, in __init__
    super(AutoVersionClient, self).__init__(*args, **kwargs)
  File "/usr/lib/python3.4/site-packages/docker/client.py", line 77, in __init__
    self._version = self._retrieve_server_version()
  File "/usr/lib/python3.4/site-packages/docker/client.py", line 97, in _retrieve_server_version
    'Error while fetching server API version: {0}'.format(e)
docker.errors.DockerException: Error while fetching server API version: ('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))