Improve referrer error message to include digest and artifact type

[Piskoo]

The error "referrer not found" during attestation operations is cryptic and doesn't indicate which artifact is missing. This makes debugging difficult when artifacts fail to upload or reference properly. Improve the error message to include the artifact digest and kind (e.g., "an artifact or piece of evidence with digest sha256:abc... (kind: HELM_CHART) can't be found").