Improve handling of protected subevents during deletion

src/pretix/control/templates/pretixcontrol/subevents/delete_bulk.html

@@ -16,6 +16,12 @@
                     </li>
                 {% endfor %}
             </ul>
+            <div class="alert-info alert">
+                {% blocktrans trimmed %}
+                    It is possible that some of the above dates can't be deleted if a plugin has data attached to them.
+                    In that case, they will be disabled instead.
+                {% endblocktrans %}
+            </div>
         {% endif %}
         {% if forbidden %}
             <p>{% blocktrans trimmed %}The following dates can't be deleted as they already have orders, but
@@ -30,7 +36,8 @@
             </ul>
         {% endif %}
         <div class="form-group submit-group">
-            <a href="{% url "control:event.subevents" organizer=request.event.organizer.slug event=request.event.slug %}" class="btn btn-default btn-cancel">
+            <a href="{% url "control:event.subevents" organizer=request.event.organizer.slug event=request.event.slug %}"
+               class="btn btn-default btn-cancel">
                 {% trans "Cancel" %}
             </a>
             <button type="submit" class="btn btn-danger btn-save" value="delete_confirm" name="action">

src/pretix/control/views/subevents.py

@@ -41,7 +41,7 @@ from django.contrib import messages
 from django.core.exceptions import ValidationError
 from django.core.files import File
 from django.db import connections, transaction
-from django.db.models import Count, F, Prefetch
+from django.db.models import Count, F, Prefetch, ProtectedError
 from django.db.models.functions import Coalesce, TruncDate, TruncTime
 from django.forms import inlineformset_factory
 from django.http import Http404, HttpResponse, HttpResponseRedirect
@@ -639,12 +639,14 @@ class SubEventBulkAction(SubEventQueryMixin, EventPermissionRequiredMixin, View)
             })
         elif request.POST.get('action') == 'delete_confirm':
             for obj in self.get_queryset():
-                if obj.allow_delete():
+                try:
+                    if not obj.allow_delete():
+                        raise ProtectedError('only deactivate', [obj])
                     CartPosition.objects.filter(addon_to__subevent=obj).delete()
                     obj.cartposition_set.all().delete()
                     obj.log_action('pretix.subevent.deleted', user=self.request.user)
                     obj.delete()
-                else:
+                except ProtectedError:
                     obj.log_action(
                         'pretix.subevent.changed', user=self.request.user, data={
                             'active': False