This is a shameless layering violation in favour of UX. It falls back
trivially to "unknown", so it's purely a UX feature.
Diagnostic sample:
```
error: hash mismatch in fixed-output derivation '/nix/store/sjfw324j4533lwnpmr5z4icpb85r63ai-x1.drv':
likely URL: https://meow.puppy.forge/puppy.tar.gz
specified: sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=
got: sha256-a1Qvp3FOOkWpL9kFHgugU1ok5UtRPSu+NwCZKbbaEro=
```
Change-Id: I873eedcf7984ab23f57a6754be00232b5cb5b02c
