Improve error message for routes that don't support custom_elements format

Problem

When a route explicitly requires _format: html (e.g. Views pages like /user/{id}/scheduled_media), accessing it via the CE API returns a 406 with the generic Drupal core
message:

Title: "Client error"
Content: "A client error happened"

This comes from core's Http4xxController::on4xx() fallback. It gives no indication of what went wrong.

The CE API should return a more descriptive error, e.g.:
- Title: "Not Acceptable"
- Content: "This page does not support the requested format. Supported formats: html."

This could be handled in CustomElementsHttpExceptionSubscriber by adding an on406() method or by improving the generic 4xx handling to include the exception message.