MDL-44468 backup: use notification API

Replace a hardcoded div.notifyproblem with a API call that,
in Standard theme, outputs the same HTML.

Also removes the .notifyproblem class from the heading,
as that might cause problems for themers who expect it to be
used consistently with the API.
MDL-44468 backup: use  html_writer::div call

Simplify the renderer code with the new html_writer
div function.