Forbid creating pages under existing Parent

This should be server-side validated. If the parent page exists already on root space, an error should be thrown since otherwise it would overwrite / mess up the old pages.
See: https://jira.xwiki.org/browse/GHIMPORT-18?focusedCommentId=106316&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-106316