Made error message more clear

The previous version did not specify why permission was denied

src/Dompdf.php

@@ -368,7 +368,7 @@ public function loadHtmlFile($file)
 
             $ext = strtolower(pathinfo($realfile, PATHINFO_EXTENSION));
             if (!in_array($ext, $this->allowedLocalFileExtensions)) {
-                throw new Exception("Permission denied on $file.");
+                throw new Exception("Permission denied on $file. This file extension is forbidden");
             }
 
             if (!$realfile) {