CloudFormationManagedUploadInfrastructure setup issues

A customer attempted to register modules using CodeBuild and reported this to us:

- The registration failed with "internal failure". It was not clear what the error was. In the end it turned out that the CodeBuild service role was missing `kms:decrypt` permissions against the KMS key used by the S3 bucket (the same KMS Key generated by the CloudFormationManagedUploadInfrastructure stack). CFN CLI should give more descriptive error messages.
- When `CloudFormationManagedUploadInfrastructure` generates, the stack should provide a "AWS::KMS::Alias" to simplify KMS key management, KMS permissions, and with making templates across many accounts.
- We should provide guidance on what the `CloudFormationManagedUploadInfrastructure` stack is and what it does. (For this point, i'm not sure if this should be explained in this repo, or in the CloudFormation user guide).