RemoveOldManualUserPages: Document and enforce required options

Follows-up dbdcceb182da5.

Current fails with unhelpful:

> PHP Fatal error:
> Call to a member function getId() on a non-object
> in GlobalCssJs/removeOldManualUserPages.php on line 23

Change-Id: Idfe430088136dd2724005583df41617df5e1ffb8