feat(tvix/nix-compat): fold NameError into Error

This being a nested error makes things more complicated than necessary.

Also, this caused BuildStorePathError to only hold NameError,
so refactor these utility functions to either return Error, or
BuildStorePathError.

Change-Id: I046fb403780cc5135df8b8833a291fc2a90fd913
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8972
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/nix-compat/src/derivation/mod.rs

@@ -212,7 +212,7 @@ impl Derivation {
                 build_output_path(derivation_or_fod_hash, output_name, &path_name).map_err(|e| {
                     DerivationError::InvalidOutputDerivationPath(
                         output_name.to_string(),
-                        store_path::BuildStorePathError::InvalidName(e),
+                        store_path::BuildStorePathError::InvalidStorePath(e),
                     )
                 })?
             };