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.
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git-svn-id: http://core.svn.wordpress.org/trunk@27495 1a063a9b-81f0-0310-95a4-ce76da25c4cd