exits with error when build succeeds if binary cache isn't available

[Readf0x]

Nom incorrectly exits with code 1 when a binary cache cannot be reached when fallback is enabled. Exiting with an error is the proper behavior when fallback isn't enabled, however, when it is nix will build whatever it can't download, in which case nom should ignore inaccessable cache errors if the build succeeds. This was causing issues with nh and likely other dependants and would interfere with scripting.

[maralorn]

Good point. This might be because sometimes nix would exit without an error code although errors had occurred. I guess though that nom shouldn’t actually modify nix behavior at all instead pass it through directly.