Superevent handler should ignore lvalert messages that do not change the FAR of an event

The superevent handler is trying to act upon lvalert messages that do not modify the FAR of the event. Here's an example from an error reported in Flower. The LVAlert packet looks like this:

{
    "object": {
        "comment": "<div style=\"white-space:pre-wrap\">WARNING:BAYESTAR:No SNR time series found, so we are creating a zero-noise SNR time series from the whitened template's autocorrelation sequence. The sky localization uncertainty may be underestimated.</div>",
        "file_version": null,
        "tag_names": [

        ],
        "file": null,
        "created": "2018-06-08T13:57:08.204469+00:00",
        "self": "https://gracedb-dev1.ligo.org/api/events/M2253/log/36",
        "issuer": "emfollow",
        "filename": "",
        "tags": "https://gracedb-dev1.ligo.org/api/events/M2253/log/36/tag/",
        "N": 36
    },
    "uid": "M2253",
    "labels": [

    ],
    "alert_type": "update",
    "file": "",
    "description": "LOG: <div style=\"white-space:pre-wrap\">WARNING:BAYESTAR:No SNR time series found, so we are creating a zero-noise SNR time series from the whitened template's autocorrelation sequence. The sky localization uncertainty may be underestimated.</div>"
}

The traceback looks like this:

Traceback (most recent call last):
  File "/home/emfollow/.local/lib/python3.6/site-packages/billiard/pool.py", line 362, in workloop
    put((READY, (job, i, result, inqW_fd)))
  File "/home/emfollow/.local/lib/python3.6/site-packages/billiard/queues.py", line 366, in put
    self.send_payload(ForkingPickler.dumps(obj))
  File "/home/emfollow/.local/lib/python3.6/site-packages/billiard/reduction.py", line 56, in dumps
    cls(buf, protocol).dump(obj)
billiard.pool.MaybeEncodingError: Error sending result: ''(1, <ExceptionInfo: HTTPError((400, \'Bad Request / b\\\'{"non_field_errors":["Event M2253 is already assigned to a Superevent"]}\\\'\'),)>, None)''. Reason: ''PicklingError("Can\'t pickle <class \'ligo.gracedb.exceptions.HTTPError\'>: it\'s not the same object as ligo.gracedb.exceptions.HTTPError",)''.

The cause of the traceback is that the server returned an HTTP Error 400, Bad Request, with the message "Event M2253 is already assigned to a Superevent".