Lots of shifting.

* Rename *Errors.toContext->toTranslationContext.
* Add a toTranslatedMessage to the *Errors modules. Pass in a translator.
* Let app-start decide how to handle the new OAuth errors, the relier shouldn't really know how to redirect.
Only use the OAuth relier/broker for verification if actually verifying for an OAuth flow... Maybe.
Remove the "home" button from the 400.html page. Fix the oauth_sign_in test.