stop iterating over tasks if you can't read the root directory

lib/storage/tasks.js

@@ -102,6 +102,7 @@ var list = Tasks.list = function (env, cb, migration) {
                     root: env.root,
                     error: e,
                 });
+                w.abort();
                 return void cb(e);
             }
             if (list.length === 0) {