Remove instances of abort(500) or raise HTTPInternalServerError

[coltonlw]

We should remove code paths in which we intentionally return a 500 status code, since this is never a desirable situation. If there really is an internal error related to the semantics of our application, a non-http exception should be raised, because the webapp2 500 error handler is only called when the exception does not inherit from HTTPException. This ensures our custom error handling / logging code is called
See #451