MDL-46148 qtype_calculatedsimple: fix notice

... when adding a dataset with a formula error.

Author: timhunt

Diff for: question/type/calculated/questiontype.php

@@ -1061,10 +1061,14 @@ public function comment_on_datasetitems($qtypeobj, $questionid, $questiontext,
         }
 
         $answers = fullclone($answers);
-        $errors = '';
         $delimiter = ': ';
         $virtualqtype =  $qtypeobj->get_virtual_qtype();
         foreach ($answers as $key => $answer) {
+            $error = qtype_calculated_find_formula_errors($answer->answer);
+            if ($error) {
+                $comment->stranswers[$key] = $error;
+                continue;
+            }
             $formula = $this->substitute_variables($answer->answer, $data);
             $formattedanswer = qtype_calculated_calculate_answer(
                 $answer->answer, $data, $answer->tolerance,