Widget Customizer: Improve error handling. First pass.

* Replace Widget_Customizer_Exception with WP_Error
* Call Previewer.cheatin() to show the cheating message if a user can't change widgets
* Call Previewer.login() to show the login form if a user is logged out
* Show a generic error message on failures

see #27419.
Built from https://develop.svn.wordpress.org/trunk@27652


git-svn-id: http://core.svn.wordpress.org/trunk@27495 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Fix 'WidgetCustomizerPreview' is undefined error in IE8/9.

* Move `WidgetCustomizerPreview` to `wp.customize.WidgetCustomizerPreview`
* Move `WidgetCustomizerPreview_exports` to export_preview_data()
* Use `_wpWidgetCustomizerPreviewSettings` to transfer settings to `wp.customize.WidgetCustomizerPreview`

see #27485.
Built from https://develop.svn.wordpress.org/trunk@27653


git-svn-id: http://core.svn.wordpress.org/trunk@27496 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Use Underscores `indexOf` for IE8 compatibility.

props adamsilverstein.
see #27485.
Built from https://develop.svn.wordpress.org/trunk@27654


git-svn-id: http://core.svn.wordpress.org/trunk@27497 1a063a9b-81f0-0310-95a4-ce76da25c4cd
General code cleanup and improving video sizing in the admin:

* Abstract the setting of a primary button and its callback in `wp.media.view.MediaFrame.MediaDetails`
* Account for the existence or non-existence of `$content_width` in the TinyMCE views for video
* Make sure video models always have dimensions, even if they are the defaults
* For browsers that are not Firefox, don't use a timeout when setting the `MediaElementPlayer` instance in TinyMCE views

See #27320.


Built from https://develop.svn.wordpress.org/trunk@27655


git-svn-id: http://core.svn.wordpress.org/trunk@27498 1a063a9b-81f0-0310-95a4-ce76da25c4cd