Only add error data to the files_not_writable WP_Error if the install already has [25775] and [25796] applied.

see #22704.

Built from https://develop.svn.wordpress.org/trunk@25797


git-svn-id: http://core.svn.wordpress.org/trunk@25709 1a063a9b-81f0-0310-95a4-ce76da25c4cd