Have axe.finishRun failed iframe state should accept serialized error object instead of null

Currently we pass `null` to `axe.finishRun` when an iframe fails for any reason on `axe.runPartial`.

```js
axe.finishRun([results, null, results])
```

However, this doesn't allow us to know why the iframe failed similar to #3002. If `axe.finishRun` were to instead accept a serialized error object we could make use of those errors (how is still being discussed).