chore: don't redirect, but show a nullResult for Left

src/Handler/Exam/AutoOccurrence.hs

@@ -152,7 +152,13 @@ postEAutoOccurrenceR tid ssh csh examn = do
     (eaofMapping, eaofAssignment, _ignoredOccurrences) <- case autoOccurrenceResult of
       (Left e) -> do
         addMessageI Error e
-        redirect $ CExamR tid ssh csh examn EUsersR
+        pure ( ExamOccurrenceMapping {
+              examOccurrenceMappingRule = examOccurrenceRule,
+              examOccurrenceMappingMapping = Map.empty
+            }
+           , Map.map (view _2) participants'
+           , eaocIgnoreRooms eaofConfig
+           )
       (Right r) -> pure r
     -- TODO use returned ignoredOccurrences
     return $ Just ExamAutoOccurrenceAcceptForm{..}