Passing error messages from SSE (Webflux) Spring Boot app to Angular 7 frontend

My application is a Spring Boot app with several end points. We are trying to add an SSE enabled endpoint, using Webflux.

Use case :

Step 1: Front-end submits a request to a POST endpoint and gets a unique ID.

Step 2: Front end fetches processed result using a GET endpoint (SSE enabled - Flux)

Angular uses EventSource object to consume the SSE endpoint. It requires the end point to produce text/event-stream. It works very well for positive test cases. But when runtime exceptions are thrown from the service, Angular frontend cannot get the HTTP status codes and exception details. It just errors with no data.

Thrown exceptions:

@ResponseStatus(code = HttpStatus.NOT_FOUND)
public class RequestNotFoundException extends RuntimeException {

    private static final long serialVersionUID = 1L;

    public RequestNotFoundException(String message) {
        super(message);
    }
}

As mentioned in this, returning a Flux<ServerSentEvent> and terminating the flux with a custom event:

Java endpoint

 @GetMapping(path= "{id}/async", produces=MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<ServerSentEvent<Optional<ProcessedResult>>> getProcessedResultStream(
        @PathVariable("id") @ApiParam(value = "The unique identifier of the request", required = true) String id)
        throws InterruptedException {
    // Combining data and adding a disconnect event to handle at the frontend.
    return Flux.concat(
            Flux.just(ServerSentEvent.<Optional<ProcessedResult>>builder(service.getProcessedResult(id))
                    .build()),
            Flux.just(ServerSentEvent.<Optional<ProcessedResult>>builder().event("disconnect").build()));
}

This helps to close the connection at the UI end by disconnecting it. But my main issue remains open. How should I go about propagating error messages back to the frontend ? All other (normal) endpoints throw custom runtime exceptions with proper HTTP codes as given above.

Should I change my ProcessedResult class to take exception details also ?

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