4xx errors from JSTOR API on assignment launch result in incorrect `mode` config in the frontend

The JSTOR assignment at https://hypothesis.instructure.com/courses/125/assignments/2852 is currently failing to load due to an authorization error when we fetch the PDF from their API. See https://hypothesis.sentry.io/issues/4464064387.

What should happen in this situation is that some kind of error is presented to the user. Instead the view navigates to `https://lms.hypothes.is/app/null` and a "Page not found" message is being shown. This URL and message is being shown because the backend is setting an invalid value for `mode` in the frontend configuration:

```json
{
"api": {
"authToken": "Bearer ..."
},
"canvas": {
},
"debug": {
"tags": [
"role:instructor"
],
"values": {
}
},
"dev": false,
"mode": null
}
```

Instead the configuration should be something that causes the frontend to render an error dialog.

Slack thread: https://hypothes-is.slack.com/archives/C02T75RKBTK/p1721106307351849?thread_ts=1721067738.484689&cid=C02T75RKBTK