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
I intended to change what is send to the StepInvocation, without changing the behaviour of describe_exception_to, so modifying the backtrace directly in filtered_exception is not an option (unless the behaviour of describe_exception_to should also be modified?).

If filtered_step_exception is defined as:
```
        def filtered_step_exception(step)
          filtered_exception.backtrace << StepBacktraceLine.new(step).to_s
        end
```
then it returns the modified backtrace array, and not the exception returned from filtered_exception (with its backtrace modified). So I do not see what to do instead.