Crash if nix-diff tries to open a derivation that doesn't exist

I have `scp`ed just a single derivation from a remote machine. If I try and diff this against a local diff, `nix-diff` is productive, but if the input derivations change, `nix-diff` seems to try and recurse into them - but that fails as I didn't copy the input derivations. It would be better if `nix-diff` could handle this and warn me that it couldn't continue doing a deep comparison.