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.

Start by reproducing the comparison with a copied derivation whose input derivations are missing, using the nix-diff command described in the issue. Trace the recursion into changed input derivations and identify where the missing derivation causes the crash. Done means the deep comparison stops gracefully and warns that it could not continue.

Written by the indexing model from the issue text.