Check if update_core() exists before calling it in Core_Upgrader. If not,...

Check if update_core() exists before calling it in Core_Upgrader. If not, issue an error message that the copy failed likely due to disk space. fixes #19883.

git-svn-id: https://core.svn.wordpress.org/trunk@20619 1a063a9b-81f0-0310-95a4-ce76da25c4cd