Check the return of wp_insert_term() for WP_Error or array. Prevents fatal...

Check the return of wp_insert_term() for WP_Error or array. Prevents fatal erros and failure to add categories when adding terms via ajax. Props mdawaffe. fixes #17938 #17939

git-svn-id: https://core.svn.wordpress.org/trunk@19792 1a063a9b-81f0-0310-95a4-ce76da25c4cd