Display full Tracebacks instead of just exception error message.

[bpw]

Seems like when student code crashes on tests, the output presentation shows just the exception message, but not the Traceback. Hopefully displaying the Traceback is just a matter of an extra method call.

Tracebacks in the output would be effective to help students get used to reading useful error messages and acting on hypotheses and evidence rather than "it didn't work, let's change this." They could just run the same test manually, but (a) a lower barrier to entry makes them more likely to try interpreting the error message and (b) sometimes there are subtle difference in environment (bindings from REPL history) that slow the manual re-testing process and confuse students.