`file_exists` also returns `FALSE` if the file is in a directory that
can't be read by the user. This e.g. happens if permissions for
`nixops(1)`-deployment keys aren't configured correctly.
This patch improves the error message for invalid files to avoid
confusion[1].
[1] https://discourse.nixos.org/t/nixops-deploy-secrets-to-nextcloud/10414/4
81662d4798