One of the production checklist items while setting up Apigee for production is handling a invalid API call. The call typically results in CLASSIFICATION_FAILURE error. This is a stackoverflow question related to issue.
Its always best to have a catch all URL in production environment of Apigee. You can create brand new proxy with a base path / . This proxy can have a single flow that returns the custom message using an assign message policy. This catch all proxy is like a custom 404 handler in web-servers.