display a friendly error message when the server is not running

[ssube]

Now that the GUI client loads the image control parameters from the server, if the server is not available, the client will not load. That means that the Github pages demo looks like a blank white screen for anyone who is not already running the API server.

That's not a great experience, so check if the server is available, and add some friendly messaging if it is not. Some parameters will be missing and queries will not load, so many dropdown menus will be empty, but the tab controls can be replaced by an error message.