Only enforce disk free space checks when doing background updates.

see #25652.

Built from https://develop.svn.wordpress.org/trunk@25869


git-svn-id: http://core.svn.wordpress.org/trunk@25781 1a063a9b-81f0-0310-95a4-ce76da25c4cd
New, better error message when we realize we won't be able to copy a file and continue with the update.

fixes #25652.

Built from https://develop.svn.wordpress.org/trunk@25870


git-svn-id: http://core.svn.wordpress.org/trunk@25782 1a063a9b-81f0-0310-95a4-ce76da25c4cd
"some files" instead of "certain files". Less stuffy.

props markjaquith.
see #25652.

Built from https://develop.svn.wordpress.org/trunk@25871


git-svn-id: http://core.svn.wordpress.org/trunk@25783 1a063a9b-81f0-0310-95a4-ce76da25c4cd