[cubicweb-ctl/fix] correctly get exception traceback_ for pdb.post_mortem
In python 3 the behavior of sys.exc_info had a very subtle change:
- in python 2 you can call if whenever you want after a try/except statement
and you'll get information about this last raise
- ipython 3, once you get out of try/except, sys.exc_info is cleaned and you'll
get (None, None, None)
Hardened the test to avoid this error from happening again.