Handle "course no longer exist" while trying to send scores with the grading bar

This seems like a strange situation to be in, how could you send launch the assigment in the first place?

Here's the error we are seeing:

```

V1.0
pox_msg_1686846739525

failure
status

999999123
replaceResult

```

https://hypothesis.sentry.io/issues/3981790495/?project=259908&query=&referrer=issue-stream&sort=freq&statsPeriod=14d&stream_index=6