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.

git-svn-id: https://develop.svn.wordpress.org/trunk@27652 602fd350-edb4-49c9-b593-d223f7449a82