Opening URLs that don't return 200 only show a blank page

We need some built-in templates for various non 200 result pages. Right now we just get a blank page and the tab heading isn't updated accordingly.

No file or test is named; start by tracing the URL-opening flow to the page-rendering entry point and inspect how non-200 responses are handled. Check existing page templates and title updates, then verify that representative non-200 responses render a built-in page instead of a blank view and update the tab heading.

Written by the indexing model from the issue text.