Return empty strings instead of WP_Errro from get_category_link(),...

Return empty strings instead of WP_Errro from get_category_link(), get_term_link(), and get_tag_link() when passed an invalid term.  Clarify phpdoc.  Props nacin. fixes #16521 for trunk

git-svn-id: https://core.svn.wordpress.org/trunk@17437 1a063a9b-81f0-0310-95a4-ce76da25c4cd