MDL-59467 core: Use hasHelper instead of catching exception

When xdebug is set to break on exceptions it stops on all exceptions,
including _caught_ exceptions.

As a result we regularly hit this uniqid helper issue if debugging with
xdebug.

The solution is to check if the helper exists using the hasHelper method
instead of catching an exception when it does not exist.

lib/outputrenderers.php

@@ -181,10 +181,10 @@ public function __construct(moodle_page $page, $target) {
     public function render_from_template($templatename, $context) {
         $mustache = $this->get_mustache();
 
-        try {
+        if ($mustache->hasHelper('uniqid')) {
             // Grab a copy of the existing helper to be restored later.
             $uniqidhelper = $mustache->getHelper('uniqid');
-        } catch (Mustache_Exception_UnknownHelperException $e) {
+        } else {
             // Helper doesn't exist.
             $uniqidhelper = null;
         }