Better frontend error page when backend is down

[ianroberts]

Current behaviour

Related to #343 - when backend is unavailable for any reason the frontend index page loads "successfully" from an HTTP perspective (i.e. the HTML page and all the associated css and JS files load correctly with 200 response codes) but when the "initizlise" RPC call fails you are left with a completely blank browser window and no error message. Navbar and footer do not appear.

You can test this by running just the frontend development server (npm run serve:frontend) without also running the backend - in this case you get the csrf_token bit of template from the index.html but nothing else.

Expected behaviour

The navbar and footer should still render correctly as it would if no user were logged in, but the page should display a sensible warning that GATE Teamware is temporarily unavailable.