errors(full: true), for nested input, could be better

## Describe the bug

```ruby
result = contract.call(params)

# errors(full: false) works fine ..
result.errors.to_h.to_json
# "{\"items\":{\"0\":{\"weight\":{\"unit\":[\"must be one of: g, kg, lb, oz\"]}}}}"

# errors(full: true), for nested input, could be better
result.errors(full: true).to_h.values.join(', ')
# "{0=\u003e{:weight=\u003e{:unit=\u003e[\"unit must be one of: g, kg, lb, oz\"]}}}"
```

I'm happy to attempt a PR, if requested.

## To Reproduce

Call a Contract that validates nested input, as shown above. If the above is unclear, I'm happy to provide more detail.

## Expected behavior

It'd be great to have better output. For example:

```
result.errors(full: true).to_h.values.join(', ')
# "items 0 weight unit must be one of: g, kg, lb, oz"
```

## My environment

- Affects my production application: No, because I guess I won't be using `errors(full: true)` in production .. :)
- Ruby version: 2.7
- OS: N/A

Start by reproducing the nested Contract case using result.errors(full: true).to_h.values.join(', ') and compare it with errors(full: false). The work is done when nested paths such as items 0 weight unit are rendered in the expected readable form, with coverage for the nested example.

Written by the indexing model from the issue text.