Add file path to sops decryption errors

[stefanprodan]

If one of the kustomization.yaml files can't be unmarshal by the SOPS decryptor, users can't tell from the error message which file was at fault e.g.

error decrypting env sources: failed to unmarshal kustomization file: error unmarshaling JSON: while decoding JSON: json: cannot unmarshal string into Go struct field Kustomization.patches of type types.Patch

This PR adds the file path to the error message.

[darkowlzz]

LGTM!