Make oembed_invalid_url return 400 instead of 404

[leedxw]

In wp-includes/class-wp-oembed-controller.php the error response for an invalid url is a 404.

                        return new WP_Error( 'oembed_invalid_url', get_status_header_desc( 404 ), array( 'status' => 404 ) );

Please consider changing this to a 400.

The oembed endpoint seems to be an absolute magnet for unauthorised vulnerability checking, and from the webserver logs we can't see the difference between oembed_invalid_url and a legitimate request that also returns a 404.

Trac ticket: https://core.trac.wordpress.org/ticket/61907