fix: return proper response for stale participant edit urls

src/Service/ParticipantFormSupportService.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Service;
 
+use App\Exception\ParticipantNotFoundException;
 use App\Form\Model\BookingDto;
 use App\Form\Model\ParticipantDto;
 use App\Form\Model\ParticipantEditDto;
@@ -24,7 +25,7 @@ class ParticipantFormSupportService
         $participant = $bookingDto->participants[$index] ?? null;
 
         if (null === $participant) {
-            throw new \InvalidArgumentException(sprintf('Participant at index %d does not exist', $index));
+            throw new ParticipantNotFoundException($index);
         }
 
         return $participant;