500 error pages render unstyled (404s are fine)

### What happened?

500s don't have any CSS

from the 🤖:
```
CKAN's 500 page only emits /restricted_theme.css from ckanext-restricted. 
None of the /webassets/* bundles (base, unaids, scheming, harvest, activity, etc.) appear. 
404 pages render with full styling, so the issue is specific to the unhandled-exception path.
```