Errors should be reported to the user instead of failing to load the page silently

[Axionyxx]

What happens

If you happen to catch the mapserver at a bad time, you might face a 503 error. Instead of this being cleanly reported, the page just remains blank.

Expected behaviour

The page throws a visible error whenever the mapserver returns a non-ok status code

Reproduction steps

1. Trigger a big render
2. Try to load the viewer set up to reach a mapserver behind a reverse proxy
3. Page doesn't load, network logs show a 503 error