Display original exception in test mode

This is very useful when running specs with `config.consider_all_requests_local = false` `config.action_dispatch.show_exceptions = true`: this displays the "real" error message that is also displayed to the user in production mode. To make debugging failing specs easier, displaying the original exception is very useful.

I don't know how to test this. If you want tests, please give me some information on how to approach this.

Thank you! :+1: