We already throw around some exceptions that are localized
(ErrorPageError and its subclasses, MalformedTitleException), but
there's no standard way to recognize them. Let's change that.
Then let's use them in the API to be able to have internationalized
errors when such exceptions are caught, instead of wrapping the
English-language version.
Change-Id: Iac7c90f92a889f8de9dae373547c07b884addaea
