S3 static site deploys do not roll back on stack failures

[farski]

In cases where an S3 static site deploy (via the custom resource) succeeds as part of a CloudFormation stack update that otherwise fails, currently the resource may not correctly handle the rollback.

(I'm actually not sure about this. CloudFormation may just send another Update event with the old parameters to trigger the rollback, in which case this should pretty much take care of itself (though perhaps #209 is more impactful in this case, because there would be new, unreleased files remaining in S3)