wp_insert_post should have true as $wp_error parameter

Add `true` as $wp_error parameter for wp_insert_post, otherwise it returns zero instead of a WP_Error object and the checkout process is not stopped.

@mikejolley - the messages from WP_Error are not displayed or logged, making debugging this difficult. Maybe we should check WP_DEBUG and display the message if it is set to true? either that or actually use the logger in WooCommerce for the first time?