views: when a document is not found report if the document doest not exist of if the user does not have access to it

fixes #4225

docbow_project/docbow/views.py

@@ -246,7 +246,16 @@ def message(request, mailbox_id, outbox=False):
     try:
         document = get_document(request, mailbox_id, outbox)
     except Document.DoesNotExist:
-        raise Http404
+        if Document.objects.filter(pk=mailbox_id):
+            messages.warning(request,
+                    _('You cannot see this document as you are not the recipient'))
+        else:
+            messages.warning(request,
+                    _('Document not found'))
+        if outbox:
+            return redirect('outbox')
+        else:
+            return redirect('inbox')
     if outbox:
         back_pair = (reverse('outbox'), gettext_noop('back to outbox'))
     else: