Make Titles with an unknown namespace ID refer to Special:Badtitle.

Without this patch, Title::getPrefixedText() would return ":Foo"
if the namespace was unknown, potentially creating a misleading
link to the main namespace. With this change, getPrefixedText()
will return something like "Special:Badtitle/NS12345:Foo".

Note that round trip behavior is broken either way.

Bug: T165149
Change-Id: I0d491a2b58ff45f207f83ee62ca6e7e6ffbf790a