Add a "server not available" 404 page to the browser preview.

[codepo8]

Is your feature request related to a problem? Please describe.

If you have a launch.json defined and you choose "launch project" and you haven't started your server, you get an empty document.

This gives me a false impression that there is a server running, but not serving the right document.

Describe the solution you'd like:

In Edge itself you get a message that the server can't be reached when you try to access localhost without starting the server.

The in-built browser should do the same.