Fix for catchable fatals thrown on invalid titles, follow-up to miscellaneous Context...

Fix for catchable fatals thrown on invalid titles, follow-up to miscellaneous Context commit pulled out of a hat (let's go for r85252).  We must always have a Title object, even on invalid titles, for the purposes of not melting the Skin.  The previous method was to make that a barely-existent half-title from SpecialPage::getTitleFor( 'Badtitle' ), where that is a special page which is not actually defined, but exists in the localisation alias lists.  Instead, subclass Title as an explicit BadTitle which we can test for.  The badtitle error page, which has empty string for its name in all forms, would probably be quite a good test of our JavaScript error checking.