Don't allow passing slugs to get_tag_link() and get_category_link(); they are...

Don't allow passing slugs to get_tag_link() and get_category_link(); they are too ambiguous.  Restore WP_Error return to get_term_link(). fixes #16521 for trunk

git-svn-id: https://core.svn.wordpress.org/trunk@17443 1a063a9b-81f0-0310-95a4-ce76da25c4cd