Volto delivers HTTP 200 when no backend server is available

[wolbernd]

Describe the bug
When no Backend Server is available opening a Volto Site in a browser delivers a web page with an appropriate error message. However the error message is delivered to the browser with a HTTP statuscode 200.

To Reproduce
Steps to reproduce the behavior:

1. Start a Volto Instance without a backend server (e.g. with docker run --name plone6-frontend --rm -e RAZZLE_API_PATH=http://localhost:8080/Plone -d -p 3000:3000 plone/plone-frontend:latest , make sure no application listens to Port 8080)
2. Open http://localhost:3000 in your browser
3. see error message about unreachable backend but page is returned with HTTP 200

Expected behavior
The call to http://localhost:3000 should have a HTTP Statuscode indicating that there is something wrong with the website (HTTP 503 or 500).

Software (please complete the following information):

• OS: Linux
• Browser: Firefox, Chrome, Curl
• Volto Version: Tested with 15.11.1 and 17.19.0 (latest plone-frontend docker image)
• Plone Version: n/a
• Plone REST API Version: n/a

[wolbernd]

I'll try to provide a PR for this, but it will probably take me until end of the week/next week until I have the time for this