The example was a worst case example. There are weaker versions thereof, for example the familiar:

    def handle(request):
      try:
          return Response(200, process(request))
      except UserError as ex:
          return Response(400, ex.message)
      except InternalError as ex:
          return Response(500)

The principle stays the same, there is not much to "handle" and no option to recover without external help, either by providing a well-formed request for 400 errors or by providing well-behaving code for 500 errors.