Admin ajax: Roll back new-style error sending in set-post-thumbnail. The JS...

Admin ajax: Roll back new-style error sending in set-post-thumbnail. The JS handlers know to parse an unrecognized response (like -1 or 0) as a failure. see #21776. see #22750.



git-svn-id: https://core.svn.wordpress.org/trunk@23058 1a063a9b-81f0-0310-95a4-ce76da25c4cd