CourseWizard: better error message on nodata failure, fixes #981

Closes #981

Merge request studip/studip!71

app/controllers/course/wizard.php

@@ -244,7 +244,8 @@ class Course_WizardController extends AuthenticatedController
         $this->stepnumber = $stepnumber;
         $this->temp_id = $temp_id;
         if (!$this->getValues()) {
-            throw new UnexpectedValueException('no data found');
+            PageLayout::postError(_('Ihre Session ist abgelaufen, bitte erneut anfangen.'));
+            $this->redirect('course/wizard');
         }
         if (isset($_SESSION['coursewizard'][$this->temp_id]['source_id'])) {
             $this->source_course = Course::find($_SESSION['coursewizard'][$this->temp_id]['source_id']);