Marking what expression evaluated to None would be extremely helpful.  So would marking the 0 denominator when there is more than one candidate: "e = a/b + c/d".  It should be easy to revise IDLE Shell's print_exception to tag the span.  In some cases, the code that goes from a traceback line to a line in a file and marks it could do the same.

What would you do when the expression is not the last line?

try:
    x/y
    ...
except Exception as e:
   ...
   raise e

The except and raise might even be in a separate module.
I look forward to the PEP and discussion.