`response.body` can be a traceback, and to date we've tried to avoid leaking tracebacks in production.

We either need to decide that leaking tracebacks in production is okay, or be smarter here about when `response.body` might be a traceback. It will be a traceback if the response we're getting here was instantiated in `aspen.algorithms.website.get_response_for_exception`, which gets run if there is an exception at any point before that in the call chain defined in `configure-aspen.py`. We can evolve Aspen to make this nicer (storing the traceback somewhere else instead of overloading `response.body`); for now we may want to find a workaround.