Link to Hub to restart stopped servers

[minrk]

When there's an error connecting to the server, JupyterLab shows a generic error:

Unlike the general jupyterlab situation, in JupyterHub there's a known link to restart the server via the hub. It would be handy to provide a link to restart the server in this dialog and provide a more JupyterHub-tailored message indicating that the user's server has been shut down.

Further, this same event may not be a connection error, it may be an error code (503 in the upcoming jupyterhub 1.0, 404 in 0.9).

This should solve jupyterhub/jupyterhub#2293

Is there an appropriate hook to customize the server connection message?

[minrk]

We should also probably be polling /api/status to check that the Hub is up, and showing a restart link if we start getting errors.