region._ensureViewIsIntact

Having the region check for view.isDestroyed does two things:

1. It allows you to handle the error of a region.show without the region killing the currentView and breaking without recourse.

2. appending isDestroyed to a Backbone.View on region empty adds the same safety for not re-showing a removed backbone view.

The extra return this; was removed to prevent jslint error for too many statements, and seemed unnecessary