Fix the error message in case of a missing realisation
Fix the error message in case of a missing realisation

Don’t say that the derivation is CA as it might happen on a non-ca
derivation too.

Technically we could always recover _something_ for a purely
input-addressed derivation (like we already do when the `ca-derivations`
xp feature isn’t enabled), but it seems better to consistently fail −
the end-result wouldn’t really make sense anyways in most cases.

NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix