Show chain of errors instead of only the most recent error

Currently Better Errors is only able to show a single error. Ruby has supported error `cause` for many years now. For example, the rescuing of an error might cause a different error to be raised. It would be very helpful to allow developers to navigate the chain of these.

As of #459 we only show the most recent error, not its cause. But this causes a problem for some errors (especially Rails ActionView templates) where the actual useful error is the second error in the chain, not the most recent.

This will require some changes to the user interface, allowing navigation between errors in the chain and focusing on the backtrace items within each.