MDL-46195 Fix fractionsaddwrong string component in lesson & qformat_webct.

I know. You should not refer to strings from other plugins, but

* qformat_webct is importing multichoice questions in that bit of code.
  I suppose a pedant would say that qformat_webct should declare dependencies
  on all the qtypes it relies on, but that would be silly.

* Lesson should be using the question engine, rather than doing its own thing.

mod/lesson/format.php

@@ -221,7 +221,7 @@ function lesson_save_question_options($question, $lesson) {
                 $totalfraction = round($totalfraction,2);
                 if ($totalfraction != 1) {
                     $totalfraction = $totalfraction * 100;
-                    $result->notice = get_string("fractionsaddwrong", "quiz", $totalfraction);
+                    $result->notice = get_string("fractionsaddwrong", "qtype_multichoice", $totalfraction);
                     return $result;
                 }
             }

question/format/webct/format.php

@@ -518,7 +518,7 @@ class qformat_webct extends qformat_default {
                                     if ($totalfraction != 1) {
                                         $totalfraction = $totalfraction * 100;
                                         $errors[] = "'{$question->name}': ".get_string('wronggrade', 'qformat_webct', $nlinecounter)
-                                                .' '.get_string('fractionsaddwrong', 'question', $totalfraction);
+                                                .' '.get_string('fractionsaddwrong', 'qtype_multichoice', $totalfraction);
                                         $questionok = false;
                                     }
                                 }