Make the "invalid authenticity token" 403 error pretty like the others

- Users *shouldn't* see this, so don't include any extra information.

![screen shot 2014-12-18 at 12 16 34](https://cloud.githubusercontent.com/assets/355033/5487962/c3702a58-86af-11e4-8a07-c7c2f1440051.png)

You've tested it separately below, so I think it's OK.

It wasn't working because views aren't rendered in controller specs by default. the `errors_controller_spec.rb` has to do this: https://github.com/alphagov/transition/blob/pretty_error_on_invalid_auth_token/spec/controllers/errors_controller_spec.rb#L7