MDL-38311 questions: manual grading API should accept commentformat

Comment format (FORMAT_...) was correctly being processed when the
manual grading happened as the result of a form submission. It was only
when done using the question_usage or question_attempt API method that
there was no way to specify the format. (Although I think the only place
this API as used was in the unit tests.)

Note that question_attempt::manual_grade API had to change, but I don't
think that is a real API change. Calling code should be using
question_usage::question_attempt, which is backwards compatible.

Note that now, if you don't pass format, then no error is generated, but
a developer debugging message is generated.

question/engine/tests/unitofwork_test.php

@@ -174,7 +174,7 @@ class question_engine_unit_of_work_test extends data_loading_method_test_base {
 
     public function test_process_action() {
 
-        $this->quba->manual_grade($this->slot, 'Acutally, that is not quite right', 0.5);
+        $this->quba->manual_grade($this->slot, 'Acutally, that is not quite right', 0.5, FORMAT_HTML);
 
         // Here, however, were we are adding a step to an existing qa, we do need to track that.
         $this->assertEquals(0, count($this->observer->get_attempts_added()));