Add more validation and error messages to CloudFormation

We need better error messages on CloudFormation. We should be able to do more verification in the task and print better error messages.

Example Scenarios:
* Trying to update a stack that is currently deleting (or is deleted)
* trying to delete a stack that is already deleted

Start by locating the CloudFormation task and its validation and error-handling entry points. Reproduce the update and delete scenarios described in the issue, then identify the existing task tests, if any. Done means invalid stack states are detected and produce clear errors instead of proceeding ambiguously.

Written by the indexing model from the issue text.