notify students for NotFoundErrors from capa_module, improve tracking

The second-most common 400 error we receive comes from NotFoundErrors in capa_module. This fix replaces the HttpResponseBadRequest with an HttpResponse with an error message for the students. If the error came from the common capa_module NotFoundError, we inform the student that the problem's state has changed since the page has been loaded and instruct the student to reload the page. If there is a different error with the ajax, we return a generic error message.

Secondly, this fix provides better tracebacks for errors that trigger ProcessingErrors

@nedbat 
@shnayder 
@wedaly