Improve error message for missing content model on import

Improve error message for missing content model on import

I was initially going to refactor out the error message into Import.php,
but it quickly became apparent that WikiImporter's error handling needs a
LOT of work. In particular, to localise the error message into the user's
language is sadly non-trivial.

Although not used currently, the MWUnknownContentModelException subclass
will help with error handling improvements in the future.

Bug: T49270
Change-Id: I9f53c9d6a8a2ea842cb2ba94d4131e10a8b08f5d