Refactor extract and gather_errors to use WP_Error methods

[alexstandiford]

As of WordPress 5.6, there are a few handy helper methods that make the helper methods extract and gather_errors potentially redundant.

https://developer.wordpress.org/reference/classes/wp_error/

It would be good to re-visit these methods, and determine if they should exist at all. If so, they probably should be refactored to use the WP_Error methods. Otherwise, they should be marked as deprecated and get removed in a future major release.