MDL-46212 questions: add {} whenver vars are put into "" strings

Having had one subtle bug cased be this, I decided to do a search and
fix all of these.

question/engine/questionusage.php

@@ -837,7 +837,7 @@ class question_usage_by_activity {
         while ($record->qubaid != $qubaid) {
             $records->next();
             if (!$records->valid()) {
-                throw new coding_exception("Question usage $qubaid not found in the database.");
+                throw new coding_exception("Question usage {$qubaid} not found in the database.");
             }
             $record = $records->current();
         }