Comments: Check if wp_new_comment() returns an error.

Adds checks throughout to allow for `wp_new_comment()` returning a `WP_Error` instance.

Updates the docs for the `pre_comment_approved` filter to include that it can be passed an error.

Props enrico.sorcinelli, ryotsun.
Fixes #39730.


Built from https://develop.svn.wordpress.org/trunk@41980


git-svn-id: http://core.svn.wordpress.org/trunk@41814 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-xmlrpc-server.php

@@ -6487,6 +6487,10 @@ class wp_xmlrpc_server extends IXR_Server {
 
 		$comment_ID = wp_new_comment($commentdata);
 
+		if ( is_wp_error( $comment_ID ) ) {
+			return $this->pingback_error( 0, $comment_ID->get_error_message() );
+		}
+
 		/**
 		 * Fires after a post pingback has been sent.
 		 *