* Only add exception/traceback on Python 3
On Python 2 the traceback could be any exception from the stack frame
and likely unrelated to the fail_json call.
On Python 3 the traceback is cleared outside any exception frame, so the
call always returns the most inner traceback (if any), and therefor is
most likely related to the fail_json call.
* Add uncertainty to traceback on Python 2
On Python 2 the last exception in the stack frame is being returned,
this could be unrelated to the actual error, especially if fail_json()
is called outside an except: block.
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