In the upgrader, only print additional error data if it is a string.

As in [25576], we sometimes want to return additional data that is not necessarily meant for UI consumption.

see #25576.

Built from https://develop.svn.wordpress.org/trunk@25775


git-svn-id: http://core.svn.wordpress.org/trunk@25688 1a063a9b-81f0-0310-95a4-ce76da25c4cd