fixed guardian login edge cases and misleading placeholders on failed loads

lib/api/marianumconnect/queries/absence/absence_prefill.dart

@@ -1,3 +1,4 @@
+import '../../../errors/app_exception.dart';
 import '../../marianumconnect_query.dart';
 import 'absence_prefill_response.dart';
 
@@ -13,3 +14,17 @@ class AbsencePrefill extends MarianumConnectQuery {
     queryParameters: {'childId': ?childId},
   );
 }
+
+/// The prefill of a guardian's report is missing a field the form cannot ask
+/// for: name and class come from the child and are not editable there, so an
+/// incomplete prefill would leave a locked, unsubmittable form behind.
+class AbsencePrefillIncompleteException extends AppException {
+  const AbsencePrefillIncompleteException({super.technicalDetails})
+    : super(
+        userMessage:
+            'Für dieses Kind sind in der Schulverwaltung noch nicht alle '
+            'Angaben hinterlegt, die für eine Abwesenheitsmeldung nötig sind. '
+            'Bitte wende dich an das Sekretariat.',
+        allowRetry: false,
+      );
+}