Reverting "extensionUtils: Remove unused property"

This reverts commit 28aa9201f0.

This broke the extension-state-changed signal.

https://bugzilla.gnome.org/show_bug.cgi?id=690173

js/misc/extensionUtils.js

@@ -132,6 +132,7 @@ function createExtensionObject(uuid, dir, type) {
     extension.type = type;
     extension.dir = dir;
     extension.path = dir.get_path();
+    extension.error = '';
     extension.hasPrefs = dir.get_child('prefs.js').query_exists(null);
 
     extensions[uuid] = extension;