Output WP_Error message when adding categories via AJAX

[ryan]

Currently, adding categories via AJAX outputs a generic error message when something goes wrong in wp_insert_category() since that function does not return WP_Error. Let's add an option to wp_insert_category() that tells it to return WP_Error instead of 0. We must maintain returning 0 by default for back-compat reasons.

[ryan]

(In [6329]) Output WP_Error message when adding categories via AJAX. fixes #5342