Auto updates: Only attempt a roll back when we've hit a critical error code.

Specifically, this means when we've failed in copying files.

see #22704.

Built from https://develop.svn.wordpress.org/trunk@25777


git-svn-id: http://core.svn.wordpress.org/trunk@25690 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Account for the possible failure of disk_free_space() in update_core().

see #22704, #25576.

Built from https://develop.svn.wordpress.org/trunk@25778


git-svn-id: http://core.svn.wordpress.org/trunk@25691 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove PHP4 compat code from the ZipArchive unzip handler, and pass the failure reason into the WP_Error return.

See #22704

Built from https://develop.svn.wordpress.org/trunk@25779


git-svn-id: http://core.svn.wordpress.org/trunk@25692 1a063a9b-81f0-0310-95a4-ce76da25c4cd