Invalid after cursor parameter returns 500 with HTML error page

## Description
Passing a malformed value to the `after` pagination cursor parameter on `/search` returns HTTP 500 with an HTML "Site unavailable" page instead of a JSON error response.

## Steps to Reproduce
GET https://catalog.data.gov/search?after=NOTAVALIDCURSOR

## Expected Behavior
HTTP 400 or 422 with a JSON error body explaining the cursor is invalid.

## Actual Behavior
HTTP 500 with an HTML page. Any client parsing the response as JSON will break with no actionable information.

## Notes
Confirmed April 27, 2026. Any malformed cursor value triggers this consistently.