Adjust error notification behavior when unable to connect to the remote jupyter server

[DonJayamanne]

• Connect to remote Jupyter server A
• Kill the jupyter server
• Reload VS Code
• Open a notebook and run a cell against the local kernel. Everything seems fine.
• At some point you're interrupted with a modal dialog with the message Failed to connect to the remote jupyter server. View Jupyter log for further details

Suggestions:

• Don't asynchronously just show a modal dialog box. We need to find a better way to notify the user when this happens. I'd say they don't need to know until they try to choose a kernel.
• Failed to fetch kernels from the remote jupyter server <name>. View Jupyter log for further details
  Where name is the Uri or the name provided for the connection (in the case of AML or the like).
• Should we also include a button Disconnect from server?

[DonJayamanne]

Solution: Display the kernels associated with the remote jupyter servers that are no longer valid. Once the user attempts to run a cell against one of these kernels, notify the user about the fact that the server is no longer recheable.