Updated secrets error message if secrets files do not exist

[aidanharan]

When you initialize a new project a .kamal/secrets file is created. If you create a new destination config config/deploy.other.yml and try to deploy you get the obscure error message ERROR (Kamal::ConfigurationError): Secret 'KAMAL_REGISTRY_PASSWORD' not found in. The error message should have been Secret 'KAMAL_REGISTRY_PASSWORD' not found, no secret files provided but there was a small bug with [] being considered true.

The original error message doesn't mention that you need to create .kamal/secrets-common or .kamal/secrets.other. This PR updates the error message so that you are made aware of the the secrets files that Kamal is looking for.