If conversion fails the action method will not be called, unless you have immediate=true on the submit button (which does not make sense anyway).
In the app raises an exception you generally can't recover anyway, this depends on the circumstances. The main reason for this is some entityManger exceptions do not allow you to resuse the entityManager, so there is not easy way to recover gracefully.
I can't really say about the back button, it depends on what makes sense for your app. If you know how you want it to work we can probably help, but what makes sense for one app does not make sense for others.