Add an incompleteSummary in addition to failureSummary

The summary property is rendered with the "fix the following" phrase, even on incomplete messages. Instead something like "check the following" would be more appropriate.

```
Check all of the following:
aria-label attribute is not well supported on a div with no valid role attribute.
```

It might also be good to deprecate the `failureSummary` property on the v2 reporter and rename it to `summary` or `issueSummary` instead. It's more appropriate for incomplete responses.