Use return value of update_post_meta() in set_post_thumbnail() instead of...

Use return value of update_post_meta() in set_post_thumbnail() instead of always returning true. Props webord. Fixes #20218.


git-svn-id: https://core.svn.wordpress.org/trunk@20172 1a063a9b-81f0-0310-95a4-ce76da25c4cd