Show warning message before deployment when a secret does not exist

[iamhopaul123]

Currently when you specify a non-existing secret in the service manifest and make a deployment, you won't get any error message until the ECS keeps failing to create any stable task, and it takes quite long time for rollback to complete. It would be nice if Copilot can validate the existence for the secrets specified and give a warning or error out if any of them does not exist.

[ashrafee-dev]

Hi! I’d love to take this on — it would be my first PR to AWS Copilot CLI 🙂. Let me know if that’s okay!