Throw a clear error when stack.yaml / stack.yaml.lock path is missing

[verifiedtm]

Summary

When the stackYaml or stackYamlLock path passed to stacklock2nix doesn't exist, Nix currently raises a bare error: path '...' does not exist from deep inside the YAML-reading IFD (read-yaml.nix), with no hint at the cause. This commonly happens when stacklock2nix is evaluated without the full repository present — e.g. a Docker build that copies only a subset of the source tree.

This guards both paths with builtins.pathExists and throws an actionable message naming the likely cause. When the paths exist they're returned unchanged, so there is no behavior change for normal builds. The check is lazy — it fires at the same point the bare error did (when the stack files are first forced), not at call time.

Test plan

• nix-instantiate --parse of the edited file passes.
• With stack.yaml present, evaluating my-example-haskell-stacklock proceeds into the YAML IFD as before (no new error).
• With a non-existent stackYaml, forcing the package set throws the new stacklock2nix: ... not found ... message instead of the bare path error.

🤖 Generated with Claude Code

[cdepillabout]

This LGTM! Thanks