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

[novemberborn]

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

For reference:

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

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

See:

ava/lib/assert.js

Line 36 in 67e4dea

class AssertionError extends Error {

Some of these properties are internal, others are outdated.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