Alerting: Use ErrImagesDone in Discord and SensuGo (#51106) (#51125)

(cherry picked from commit 7235480be5)

Co-authored-by: George Robinson <george.robinson@grafana.com>

pkg/services/ngalert/notifier/channels/discord.go

@@ -201,8 +201,7 @@ func (d DiscordNotifier) constructAttachments(ctx context.Context, as []*types.A
 	_ = withStoredImages(ctx, d.log, d.images,
 		func(index int, image *ngmodels.Image) error {
 			if embedQuota < 1 {
-				// TODO: Could be a sentinel error to stop execution.
-				return nil
+				return ErrImagesDone
 			}
 
 			if image == nil {

pkg/services/ngalert/notifier/channels/sensugo.go

@@ -147,6 +147,7 @@ func (sn *SensuGoNotifier) Notify(ctx context.Context, as ...*types.Alert) (bool
 			// one image per request.
 			if image != nil && image.URL != "" && imageURL == "" {
 				imageURL = image.URL
+				return ErrImagesDone
 			}
 			return nil
 		}, as...)