issue #471: revert exception and add some comments

Author: dmitriim

classes/local/store/object_file_system.php

@@ -1003,14 +1003,15 @@ public function cron() {
     }
 
     /**
-     * Object fs doesn't use trashdir.
+     * Object fs doens't use trashdir - trigger exception.
      *
      * @param string $contenthash The content hash
      * @return string The full path to the trash directory
      */
     protected function get_trash_fulldir_from_hash($contenthash) {
-        debugging('Objectfs does not implement a trashdir.');
-        return '';
+        // Nothing should be asking objectstorage for it's trashdir path - it doesn't exist.
+        // However let's throw an exception unless something else comes up that suggests we need to change it.
+        throw new \coding_exception('Objectfs does not implement a trashdir.');
     }
 
     /**
@@ -1020,6 +1021,8 @@ protected function get_trash_fulldir_from_hash($contenthash) {
      * @return string The full path to the trash file
      */
     protected function get_trash_fullpath_from_hash($contenthash) {
+        // Method get_trash_fullpath_from_hash called by core filelib functions,
+        // and we can't easily change those across stables. So add debugging here.
         debugging('Objectfs does not implement a trashdir.');
         return '';
     }