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: http://core.svn.wordpress.org/trunk@24732 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Compress scripts/styles: 3.6-RC1-24733.

git-svn-id: http://core.svn.wordpress.org/trunk@24733 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fixes and tweaks for the metabox accordion.

* Fixed hover issues in Customizer.
* Toned down the default active/hover style (as it is normally not the top level menu, and should not be so prominent).
* Allow the active section to be closed.
* Other misc fixes.

Props ocean90, helen. Fixes #23449.

git-svn-id: http://core.svn.wordpress.org/trunk@24734 1a063a9b-81f0-0310-95a4-ce76da25c4cd