BUG: Fix CMS forms with validation errors responding incorrectly.

A new form instance was being constructed for the response, which mean
that a lot of the validation information was lost. This fix means that:

* Enterered data is correctly persisted.
* A validation error notification is displayed.
FIX: Fix not switching to the correct tab on validation error.
Merge pull request #2514 from ajshort/pull-4

CMS Validation