Exception throwing

Signed-off-by: Edward Z. Yang <ezyang@meta.com>

library/HTMLPurifier/HTMLDefinition.php

@@ -264,9 +264,8 @@ class HTMLPurifier_HTMLDefinition extends HTMLPurifier_Definition
         if (isset($this->info_content_sets['Block'][$block_wrapper])) {
             $this->info_block_wrapper = $block_wrapper;
         } else {
-            trigger_error(
+            throw new Exception(
                 'Cannot use non-block element as block wrapper',
-                E_USER_ERROR
             );
         }