fix: Show descriptive message for 406 errors instead of generic text

Wolfgang Zieglerrequested to merge
issue/lupus_ce_renderer-3579385:3579385-improve-error-message into 2.x

When a route requires _format: html and is accessed via custom_elements format, Drupal returns a 406. The error page previously showed the generic "A client error happened" from core's Http4xxController.

Now shows "This page is not available in custom elements format." for 406 errors. Also fixes reading _exception_statuscode from query params instead of request attributes (where core sets it).

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Closes #3579385

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