MDL-37374 questions: use property_exists rather than isset

$a->field = null; isset($a->field) returns false, which is typical PHP.
I also improve the error handling a bit.

question/type/edit_question_form.php

@@ -481,7 +481,7 @@ abstract class question_edit_form extends question_wizard_form {
         if (is_array($extraquestionfields) && !empty($question->options)) {
             array_shift($extraquestionfields);
             foreach ($extraquestionfields as $field) {
-                if (isset($question->options->$field)) {
+                if (property_exists($question->options->$field)) {
                     $question->$field = $question->options->$field;
                 }
             }

question/type/questiontypebase.php

@@ -454,21 +454,12 @@ class question_type {
                 $options->$questionidcolname = $question->id;
             }
             foreach ($extraquestionfields as $field) {
-                if (!isset($question->$field)) {
-                    $result = new stdClass();
-                    $result->error = "No data for field $field when saving " .
-                            $this->name() . ' question id ' . $question->id;
-                    return $result;
+                if (property_exists($question->$field)) {
+                    $options->$field = $question->$field;
                 }
-                $options->$field = $question->$field;
             }
 
-            if (!$DB->{$function}($question_extension_table, $options)) {
-                $result = new stdClass();
-                $result->error = 'Could not save question options for ' .
-                        $this->name() . ' question id ' . $question->id;
-                return $result;
-            }
+            $DB->{$function}($question_extension_table, $options);
         }
 
         $extraanswerfields = $this->extra_answer_fields();