Notify administrators of successful, failed, and pending core updates.

Blocks future background updates after critical failures, but allow retries in certain situations. More in the ticket.

fixes #10787.

Built from https://develop.svn.wordpress.org/trunk@25841


git-svn-id: http://core.svn.wordpress.org/trunk@25753 1a063a9b-81f0-0310-95a4-ce76da25c4cd