Check if the destination space for publication is a terminal page

If the destination space of the publication has been wrongly set to a terminal page, it would create errors at publication (because no sub-pages could be moved to it).

This should be checked at the beginning of the publication process.

Note: this shouldn't happen from a user point of view, as the picker of the publication configuration selects only spaces. It could happen while using the API directly. This is then a minor issue for now.