Make reduce -n more sensible

[sophiajt]

Description

This slightly changes the semantics for reduce -n. Now, we don't add the index to the $acc on the way out if we're in numbered mode. This is so that the $acc variable keeps the same type through the process, which is a bit easier to read and is more predictable.

I also went ahead and made errors during reduction immediate bubble up and added ctrlc protection to the reduction.

fixes #4759

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass