Improve clean-aws.sh output and handling of AccountSharedResources

[tlindsay42]

While working with @DarwinJS today and running make clean-aws in his test account to destroy some dangling resources, we found that AccountSharedResources was failing to delete due to an managed policy attached to the IAM Role, but the error message for this was not intuitive and could use improvement.

Waiter StackDeleteComplete failed: Waiter encountered a terminal failure state: For expression "DELETE_FAILED"...