Only enforce disk free space checks when doing background updates.

see #25652.


git-svn-id: https://develop.svn.wordpress.org/trunk@25869 602fd350-edb4-49c9-b593-d223f7449a82
New, better error message when we realize we won't be able to copy a file and continue with the update.

fixes #25652.


git-svn-id: https://develop.svn.wordpress.org/trunk@25870 602fd350-edb4-49c9-b593-d223f7449a82