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.