Return error on stream error for retry (#565)

`handleEvent` was handling the an `error` type by printing something,
but then not continuing to raise the error.

This means that in a stream error, particuarly of type INTERNAL_ERROR,
the backoff/retry mechanism never took place as the return was `nil` and
the agent just stopped.

The problem is particularly relevant with Google Load Balancers and
their handling of http2/websocket long lived connections:
golang/go#51323

---------

Co-authored-by: Louis Garman <[email protected]>

internal/agent/agent.go

@@ -116,7 +116,10 @@ func (a *agent) Start(ctx context.Context) error {
 
 	g.Go(func() error {
 		if err := a.spooler.start(ctx); err != nil {
-			return fmt.Errorf("spooler terminated: %w", err)
+			// only report error if context has not been canceled
+			if ctx.Err() == nil {
+				return fmt.Errorf("spooler terminated: %w", err)
+			}
 		}
 		return nil
 	})

internal/agent/spooler.go

@@ -110,20 +110,25 @@ func (s *spoolerDaemon) reinitialize(ctx context.Context) error {
 	}
 	// then spool events as they come in
 	for event := range sub {
-		s.handleEvent(event)
+		err = s.handleEvent(event)
+		if err != nil {
+			return err
+		}
 	}
 	return nil
 }
 
-func (s *spoolerDaemon) handleEvent(ev pubsub.Event) {
+func (s *spoolerDaemon) handleEvent(ev pubsub.Event) error {
 	switch payload := ev.Payload.(type) {
 	case *run.Run:
 		s.handleRun(ev.Type, payload)
 	case string:
 		s.Info("stream update", "info", string(payload))
 	case error:
 		s.Error(payload, "stream update")
+		return payload
 	}
+	return nil
 }
 
 func (s *spoolerDaemon) handleRun(event pubsub.EventType, run *run.Run) {