Surface error in place of empty summary when the API responds with an error

[cjcenizal]

In some cases the prompt length exceeds the limit, resulting in the API returning this error:

"status": [
    {
        "code": "BAD_REQUEST",
        "statusDetail": "Too much text for the summarizer to summarize. Please try reducing the number of search results to summarize, or the context of each search result.",
        "cause": null
    }
],

The result is the summary shows up as empty, which is confusing and unhelpful:

We should update the UI to show a generic error here instead.

[3eif]

Should be addressed in #53