Display a form error rather than a page error if a guest is already invited

apps/activity/admin.py

@@ -3,6 +3,7 @@
 
 from django.contrib import admin
 from note_kfet.admin import admin_site
+from .forms import GuestForm
 
 from .models import Activity, ActivityType, Entry, Guest
 
@@ -35,6 +36,7 @@ class GuestAdmin(admin.ModelAdmin):
     Admin customisation for Guest
     """
     list_display = ('last_name', 'first_name', 'activity', 'inviter')
+    form = GuestForm
 
 
 @admin.register(Entry, site=admin_site)