I haven't tried this (I use single EC2 deployments, some Heroku, also have a Hetzner server) but it is something that I have been thinking of: have the web services that back up your web app on a single server, and yes that will fail on hopefully rare occasions. Host the Javascript+HTML5+CSS front end on S3 with Cloudfront CDN. The home page of your app will almost never go offline and you control what to report to your users if your backend services are offline. Sure you lose core functionality, but you still have static content and a friendly message about temporary lack of services.
Going beyond that at a cost of slow response times when trying to access a downed backend, you could deploy back end web services to two different hosting providers, perhaps running something like CouchDB replicated on each provider. The Javascript on your UI could switch to an alternative back end after a timeout. For "one page" style apps, you could maintain the state information that a backend host is down in the browser.
Going beyond that at a cost of slow response times when trying to access a downed backend, you could deploy back end web services to two different hosting providers, perhaps running something like CouchDB replicated on each provider. The Javascript on your UI could switch to an alternative back end after a timeout. For "one page" style apps, you could maintain the state information that a backend host is down in the browser.