Offer users better error if fstab entries exist

[Hoverbear]

Description

Due to #37 we can't really see if an apfs volume would be created, so we can't know if the fstab entry is already existing an correct (since creating an apfs volume would change the UUID and require the fstab to be updated). So we can only offer a nice error for now.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[cole-h]

LGTM after these last nits.