* Introduce a wp_checkdate() function with a single filter to centralise the code that validates dates.
* Improve the error message
* Correctly handle the return value of wp_insert_post which is not always a WP_Error on failure
Fixes#17180
git-svn-id: https://develop.svn.wordpress.org/trunk@21922 602fd350-edb4-49c9-b593-d223f7449a82