[Bug] fetchEventSource need logic to handle non-streaming responses

[yanCode]

to reproduce this issue, you can disconnect your newwork on your computer, then send anything as a question, you can see there is an endless loading icon in 1, while it's expected to directly inform the network has a problem in 2.

recently import { fetchEventSource} from "@microsoft/fetch-event-source";
has been included to handle the stream response returned from open api endpoint.
as I checked the official doc, https://github.com/Azure/fetch-event-source you can see it requires extra logic to handle if a response is not of type EventStreamContentType .

as in route.ts we do we have logic

console.error("[OpenAI] ", e);
    return NextResponse.json(prettyObject(e));

returning a non-streaming response.

I am preparing a PR now.