[feature] handle it more gracefully when notebook cannot reach KFP

If a notebook is opened when KFP is not connected, a generic error appears. We need to handle this more gracefully.

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When the Error is thrown because the KFP server is not connected, it should say:
`Error: KFP is not reachable.`
`You can still compile notebooks, but you cannot upload or run pipelines.`
`You can find more information under /opt/app-root/src/kale.log`

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For context, when I replaced my normal kfp host url with `localhost=55555`, this was the error in the kale log:
`urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='localhost', port=55555): Max retries exceeded with url: /apis/v2beta1/experiments?page_token=&page_size=10&sort_by=&namespace=kale (Caused by NewConnectionError("HTTPSConnection(host='localhost', port=55555): Failed to establish a new connection: [Errno 111] Connection refused"))`