Make errors non-exhaustive

[cole-h]

Description

Adding a new error variant shouldn't force us to bump the version due to being semver-incompatible.

Done with the following:

rg 'enum .+Error.* \{' --no-heading -n | cut -d: -f1-2 | while read file; hx "$file"; end

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