FIX Throw error if trying to add a table-less DataObject class to index

Its impossible for SearchUpdater#handle_manipulation to figure out the difference
between writing to a table-less class (like Page if theres no $db set) and the
table-having parent (like SiteTree) because it only examines the DB manipulation.

This meant if you tried to index Page (again assuming there are no $db fields on Page), only subclasses that did have $db fields would be indexed in realtime.

We cant fix, but we can throw an error if you try to do that.