hint user that docker might not be running in renku session commands

[Panaetius]

Right now, if the docker service isn't running or there's some issue with it, we just get an error like

Traceback (most recent call last):
  File “[...]/site-packages/urllib3/connectionpool.py”, line 449, in _make_request
    six.raise_from(e, None)
  File “<string>“, line 3, in raise_from
  File “[...]/site-packages/urllib3/connectionpool.py”, line 444, in _make_request
    httplib_response = conn.getresponse()
  File “[...]/http/client.py”, line 1377, in getresponse
    response.begin()
  File “[...]/http/client.py”, line 320, in begin
    version, status, reason = self._read_status()
  File “[...]/http/client.py”, line 281, in _read_status
    line = str(self.fp.readline(_MAXLINE + 1), “iso-8859-1”)
  File “[...]/socket.py”, line 704, in readinto
    return self._sock.recv_into(b)
socket.timeout: timed out
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File “[...]/site-packages/requests/adapters.py”, line 440, in send
    resp = conn.urlopen(
  File “[...]/site-packages/urllib3/connectionpool.py”, line 785, in urlopen
    retries = retries.increment(
  File “[...]/site-packages/urllib3/util/retry.py”, line 550, in increment
    raise six.reraise(type(error), error, _stacktrace)
  File “[...]/site-packages/urllib3/packages/six.py”, line 770, in reraise
    raise value
  File “[...]/site-packages/urllib3/connectionpool.py”, line 703, in urlopen
    httplib_response = self._make_request(
  File “[...]/site-packages/urllib3/connectionpool.py”, line 451, in _make_request
    self._raise_timeout(err=e, url=url, timeout_value=read_timeout)
  File “[...]/site-packages/urllib3/connectionpool.py”, line 340, in _raise_timeout
    raise ReadTimeoutError(
urllib3.exceptions.ReadTimeoutError: UnixHTTPConnectionPool(host=‘localhost’, port=None): Read timed out. (read timeout=60)
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File “[...]/site-packages/renku/ui/cli/exception_handler.py”, line 133, in main
    return super().main(*args, **kwargs)
  File “[...]/site-packages/renku/ui/cli/exception_handler.py”, line 92, in main
    return super().main(*args, **kwargs)
  File “[...]/site-packages/click/core.py”, line 1055, in main
    rv = self.invoke(ctx)
  File “[...]/site-packages/click/core.py”, line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File “[...]/site-packages/click/core.py”, line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File “[...]/site-packages/click/core.py”, line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File “[...]/site-packages/click/core.py”, line 760, in invoke
    return __callback(*args, **kwargs)
  File “[...]/site-packages/renku/ui/cli/session.py”, line 222, in start
    session_start_command()
  File “[...]/site-packages/renku/command/command_builder/command.py”, line 252, in execute
    output = self._operation(*args, **kwargs)  # type: ignore
  File “pydantic/decorator.py”, line 40, in pydantic.decorator.validate_arguments.validate.wrapper_function
  File “pydantic/decorator.py”, line 134, in pydantic.decorator.ValidatedFunction.call
  File “pydantic/decorator.py”, line 206, in pydantic.decorator.ValidatedFunction.execute
  File “[...]/site-packages/renku/core/session/session.py”, line 118, in session_start
    session_name = provider_api.session_start(
  File “[...]/site-packages/renku/core/session/docker.py”, line 187, in session_start
    container = self.docker_client().containers.run(
  File “[...]/site-packages/docker/models/containers.py”, line 826, in run
    container.start()
  File “[...]/site-packages/docker/models/containers.py”, line 404, in start
    return self.client.api.start(self.id, **kwargs)
  File “[...]/site-packages/docker/utils/decorators.py”, line 19, in wrapped
    return f(self, resource_id, *args, **kwargs)
  File “[...]/site-packages/docker/api/container.py”, line 1108, in start
    res = self._post(url)
  File “[...]/site-packages/docker/utils/decorators.py”, line 46, in inner
    return f(self, *args, **kwargs)
  File “[...]/site-packages/docker/api/client.py”, line 233, in _post
    return self.post(url, **self._set_request_timeout(kwargs))
  File “[...]/site-packages/requests/sessions.py”, line 577, in post
    return self.request(‘POST’, url, data=data, json=json, **kwargs)
  File “[...]/site-packages/requests/sessions.py”, line 529, in request
    resp = self.send(prep, **send_kwargs)
  File “[...]/site-packages/requests/sessions.py”, line 645, in send
    r = adapter.send(request, **kwargs)
  File “[...]/site-packages/requests/adapters.py”, line 532, in send
    raise ReadTimeout(e, request=request)
requests.exceptions.ReadTimeout: UnixHTTPConnectionPool(host=‘localhost’, port=None): Read timed out. (read timeout=60)

Where the localhost connection that fails is to the docker service api.

We should catch this and reraise with a human readable error, along the lines of "Couldn't connect to docker service. Please make sure it is running and up to date"

[Panaetius]

Can be done together with #3264