Confusing error message when trusted substitutor key is not valid base64

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When adding a trusted substitutor key that isn't valid base64 (for example by accidentally typoing it) the resulting error message only mentions "Invalid base64 string", with no context as to where the issue is originating.

Steps To Reproduce

1. Add new substitutor key in /etc/nix/nix.conf
2. Typo it in some way (make it invalid base64)
3. Run nix-shell -p hello (or similar)
4. See error: "Invalid base64 string"

Expected behavior

A clear and concise error message.