CloudFormation: Scan for missing S3 artifacts

[nmoutschen]

Some templates might use S3 artifact links, which might not be available.

It'd be nice to be able to scan for those.

[marekq]

Thanks for the great suggestion - I hope to have some sort of fix for this in place soon. I need to figure out better how to collect the various forms of S3 objects, the check itself is pretty straight forward.