Display pages that failed to load

Pages may fail to load and will be [skipped](https://github.com/BeaconCMS/beacon/issues/367), displaying a 404 error page in its place.

Such pages should display a `Failed` status in the Pages (/pages) interface with the stack trace that caused such page to fail loading.

Start with the page-loading flow associated with issue #367 and the Pages (/pages) interface. Trace where failed loads become skipped pages and a 404, then determine how the interface can expose a Failed status and stack trace. Done means failed pages appear in /pages with that status and trace instead of only a 404.

Written by the indexing model from the issue text.