Fix error backtrace for the JSON formatter output

@wip-new-core removed for three scenarios in the json_formatter.feature
The step backtrace 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 backtrace line when the StepInvocation
is created.
A DocString object cannot be raised as en exception in JRuby
Make the modified backtrace not leak to @exception

The modification of the backtrace in filtered_step_exception should not
affect the backtrace of @exception.
Merge pull request #704 from brasmusson/json-formatter-error-backtrace

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