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).

Comienza trazando el flujo de registro de CFN CLI y el stack CloudFormationManagedUploadInfrastructure mencionado en el informe. Comprueba cómo se exponen los fallos internos y cómo se genera la clave KMS; después, revisa la pregunta de la guía del usuario de CloudFormation. Se considerará terminado cuando las indicaciones de error solicitadas, el alias de KMS y la explicación del stack tengan responsables claros y cambios correspondientes en la implementación o la documentación.

Escrito por el modelo de indexación a partir del texto del issue.