Improve error handling. Support additional (empty) menu manager for e…

…rror context.

Because if context is an error instance we are not able to find the correct context for build the url.

src/z3c/menu/ready2go/manager.py

@@ -26,3 +26,15 @@ class MenuManager(manager.ConditionalViewletManager):
     """Menu manager for all kind of menu items"""
 
     zope.interface.implements(interfaces.IMenuManager)
+
+
+class EmptyMenuManager(object):
+    """Empty menu manager."""
+
+    zope.interface.implements(interfaces.IMenuManager)
+
+    def update(self):
+        pass
+
+    def render(self):
+        return u''