Return false from wp_delete_post() and wp_delete_attachment() if the DELETE is unsuccessful.

This prevents attachments from being deleted off disk when a DB is locked and the delete is otherwise unsuccessful.

fixes #25107.

Built from https://develop.svn.wordpress.org/trunk@26543


git-svn-id: http://core.svn.wordpress.org/trunk@26435 1a063a9b-81f0-0310-95a4-ce76da25c4cd