Abort connection instead of flushing committed response data when throwing an Application Exception in Kestrel

After introducing `HttpResponse.StartAsync()`, if someone does the following:
```c#
await httpContext.Response.StartAsync();
throw new Exception();
```

This shouldn't write any data to the client. However, today it does because we call PipeWriter.Complete() if we are either Aborting or Disposing the OutputProducer, rather than PipeWriter.Complete(exception).

I tried to do this as part of https://github.com/aspnet/AspNetCore/pull/6967, but it was a much larger change than expected.