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
but you do not create a duplicate of filtered_exception, or a I missing something?