wip-new-core: Fix error backtrace for the JSON formatter output

The step back trace in not only needed for the print_exception method in StepInvocation, but also for the before_step_result/after_step_result methods. Therefore add the step back trace line when the StepInvocation
is created.

@wip-new-core removed for three scenarios in the json_formatter.feature
Oh, I think a misunderstood your question, you were asking if the line should instead be:
```
  exception = filtered_exception.dup
```

First I was thinking that because the filtered_exception method is performing ```new```, that would not be necessary. 

I did explore how the backtrace of ```@exception``` was affected, and indeed it is modified by the modification of ```filtered_step_exception```. And it still is if ```.dup``` or ```.clone``` is applied to ```filtered_exception``` (as above). To make the backtrace of ```@exception``` unaffected, I had to apply ```.dup``` on ```@exception``` in the ```filtered_exception``` method.