[FR] Add helpful error message when using the App in a remote notebook without appropriate port forwarding

When using the FiftyOne App in a remote Jupyter notebook, you need to forward the port used by the App (say YYYY):

# On local machine
ssh -N -L 5151:localhost:YYYY [<username>@]<hostname>

so that when you launch the App in a notebook cell:

# Launch the App in a notebook cell
session = fo.launch_app(dataset, port=YYYY)

you will see the App as expected.

However, if you don't forward the port, you'll see this when launching the App:

It would be helpful to the user if we detected this case and printed a helpful error message instructing the user how to forward the port and pointing them to documentation on the subject.