fix: return proper response for stale participant edit urls

tests/Service/ParticipantFormSupportServiceTest.php

@@ -6,6 +6,7 @@ namespace App\Tests\Service;
 
 use App\BusProNet\Model\Booking;
 use App\BusProNet\Model\Travel;
+use App\Exception\ParticipantNotFoundException;
 use App\Form\Model\BookingDto;
 use App\Form\Model\ParticipantDto;
 use App\Service\ParticipantFormSupportService;
@@ -19,7 +20,7 @@ class ParticipantFormSupportServiceTest extends TestCase
         $service = $this->createService();
         $bookingDto = $this->createBookingDto();
 
-        $this->expectException(\InvalidArgumentException::class);
+        $this->expectException(ParticipantNotFoundException::class);
         $this->expectExceptionMessage('Participant at index 2 does not exist');
 
         $service->ensureParticipantExists($bookingDto, 2);