Posting: Improve the invalid date protection code based on feedback from nacin.

* 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

wp-includes/functions.php

@@ -3703,3 +3703,13 @@ function _device_can_upload() {
 	return true;
 }
 
+/**
+ * Test if the supplied date is valid for the Gregorian calendar
+ *
+ * @since 3.5.0
+ *
+ * @return bool true|false
+ */
+function wp_checkdate( $month, $day, $year, $source_date ) {
+	return apply_filters( 'wp_checkdate', checkdate( $month, $day, $year ), $source_date );
+}