* Shows an error message if the current diff can't be loaded.
* For bulk pre-loading, catches errors, and cuts subsequent requests in half, until eventually giving up.
* Some CSS fixes related to this, and the loading spinner.
* `wp.revisions.loadAll()` now returns a promise representing whether or not all revisions could be loaded.
Fixes#24758.
git-svn-id: https://develop.svn.wordpress.org/trunk@24732 602fd350-edb4-49c9-b593-d223f7449a82