Check this out: https://developer.amazon.com/alexa-voice-service - I think it fits your use case better than using an Alexa Skill.
I was experimenting something with user exceptions - which I would not recommend anyway - but since you are exposing a REST API the module's Exception Handler will not catch any exception.
Besides that you, have to know where your user is in the app in order to control navigation and an Alexa skill is probably not the easiest or safest way to do it.