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.

backup/util/ui/renderer.php

@@ -222,8 +222,8 @@ public function backup_details_nonstandard($nextstageurl, array $details) {
     public function backup_details_unknown(moodle_url $nextstageurl) {
 
         $html  = html_writer::start_tag('div', array('class' => 'unknownformat'));
-        $html .= $this->output->heading(get_string('errorinvalidformat', 'backup'), 2, 'notifyproblem');
-        $html .= html_writer::tag('div', get_string('errorinvalidformatinfo', 'backup'), array('class' => 'notifyproblem'));
+        $html .= $this->output->heading(get_string('errorinvalidformat', 'backup'), 2);
+        $html .= $this->output->notification(get_string('errorinvalidformatinfo', 'backup'), 'notifyproblem');
         $html .= $this->output->single_button($nextstageurl, get_string('continue'), 'post');
         $html .= html_writer::end_tag('div');