Fix exception in Import, when import of a revision fails

Fix exception in Import, when import of a revision fails

A 'notice' is thrown when an import fails, for some reason,
such as the user does not have permission, and the reason
is reported to the user.

In this case, $title is false and not a Title object,
as needed by the beforeImportPage callback (which calls
WikiPage::factory).  As well, $pageInfo['_title'] is undefined,
in pageOutCallback, which also calls WikiPage::factory via
finishImportPage.

Bug: T108544
Change-Id: I55042fdf305cd1198d3a4b28a0ebb5ce31b76a1f