Officially expose assertion error properties on `t.try()` results

#1947 landed a new `t.try()` assertion. We haven't yet figured out what assertion error details we want to expose.

For reference:

```js
const attempt = await t.try(tt => {
tt.is(true, false)
})

const [error] = attempt.errors
// What should we define on this `error` object?
```

See: https://github.com/avajs/ava/blob/67e4deac2857503e5fac7d38da2d23014eb29724/lib/assert.js#L36

Some of these properties are internal, others are outdated. So let's hash out which ones we're comfortable exposing and supporting.

Start by reading lib/assert.js around line 36 and the existing t.try() behavior introduced in #1947. Review which assertion error properties are currently present, then determine which details should be officially exposed and supported on attempt.errors. Done means the public error shape has been agreed and documented or tested.

Written by the indexing model from the issue text.