MDL-59606 quiz responses: fix edge cases in the report

This patch conbines valuable contributions from Kashmira Nagwekar and
Luca Bösch. Many thanks to them. However, the final form of the fix,
and hence the blame, falls to me -- Tim.

There were several issues here:

* The load_questions_usages_by_activity method in
  question/engine/datalib.php was incorrectly treating the case
  when no data was returned. (Looks like a historic copy-pase from
  other methods that fetch one item by unique id, which therefore
  must exist.)

* The report was not correctly handling the display when the 'Which
  tries' was set to 'with, and without, attempts'.

* It was possible to select the 'All tries' option when also saying
  'Users without attempts'. This combination makes not sense, so
  a disabledIf rule was added to the form.

question/engine/datalib.php

@@ -525,15 +525,11 @@ ORDER BY
     qas.sequencenumber
     ", $qubaids->usage_id_in_params());
 
-        if (!$records->valid()) {
-            throw new coding_exception('Failed to load questions_usages_by_activity for qubaid_condition :' . $qubaids);
-        }
-
         $qubas = array();
-        do {
+        while ($records->valid()) {
             $record = $records->current();
             $qubas[$record->qubaid] = question_usage_by_activity::load_from_records($records, $record->qubaid);
-        } while ($records->valid());
+        }
 
         $records->close();