Revisions: Better error handling.

* 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://core.svn.wordpress.org/trunk@24732 1a063a9b-81f0-0310-95a4-ce76da25c4cd