isolate the actual error from this query

I couldn't the exact error, so this is the best approximation i could come up with.

src/internal/m365/collection/exchange/events_backup.go

@@ -2,6 +2,7 @@ package exchange
 
 import (
 	"errors"
+	"net/http"
 	"slices"
 
 	"github.com/alcionai/clues"
@@ -60,13 +61,21 @@ func (h eventBackupHandler) NewContainerCache(
 	}
 }
 
+// todo: this could be further improved buy specifying the call source and matching that
+// with the expected error.  Might be necessary if we use this for more than one error.
+// But since we only call this in a single place at this time, that additional guard isn't
+// built into the func.
 func (h eventBackupHandler) CanSkipItemFailure(
 	err error,
 	resourceID, itemID string,
 	opts control.Options,
 ) (fault.SkipCause, bool) {
-	// yes, this is intentionally a todo.  I'll get back to it.
-	if !errors.Is(err, clues.New("todo fix me")) {
+	if err == nil {
+		return "", false
+	}
+
+	if !errors.Is(err, graph.ErrServiceUnavailableEmptyResp) &&
+		!clues.HasLabel(err, graph.LabelStatus(http.StatusServiceUnavailable)) {
 		return "", false
 	}
 

src/internal/m365/collection/exchange/events_backup_test.go

@@ -1,6 +1,7 @@
 package exchange
 
 import (
+	"net/http"
 	"testing"
 
 	"github.com/alcionai/clues"
@@ -12,6 +13,7 @@ import (
 	"github.com/alcionai/corso/src/pkg/control"
 	"github.com/alcionai/corso/src/pkg/fault"
 	"github.com/alcionai/corso/src/pkg/services/m365/api"
+	"github.com/alcionai/corso/src/pkg/services/m365/api/graph"
 )
 
 type EventsBackupHandlerUnitSuite struct {
@@ -61,7 +63,7 @@ func (suite *EventsBackupHandlerUnitSuite) TestHandler_CanSkipItemFailure() {
 		},
 		{
 			name: "non-matching resource",
-			err:  clues.New("fix me I'm wrong"),
+			err:  graph.ErrServiceUnavailableEmptyResp,
 			opts: control.Options{
 				SkipTheseEventsOnInstance503: map[string][]string{
 					"foo": {"bar", "baz"},
@@ -71,17 +73,31 @@ func (suite *EventsBackupHandlerUnitSuite) TestHandler_CanSkipItemFailure() {
 		},
 		{
 			name: "non-matching item",
-			err:  clues.New("fix me I'm wrong"),
+			err:  graph.ErrServiceUnavailableEmptyResp,
 			opts: control.Options{
 				SkipTheseEventsOnInstance503: map[string][]string{
 					resourceID: {"bar", "baz"},
 				},
 			},
 			expect: assert.False,
+			// the item won't match, but we still return this as the cause
+			expectCause: fault.SkipKnownEventInstance503s,
+		},
+		{
+			name: "match on instance 503 empty resp",
+			err:  graph.ErrServiceUnavailableEmptyResp,
+			opts: control.Options{
+				SkipTheseEventsOnInstance503: map[string][]string{
+					resourceID: {"bar", itemID},
+				},
+			},
+			expect:      assert.True,
+			expectCause: fault.SkipKnownEventInstance503s,
 		},
 		{
 			name: "match on instance 503",
-			err:  clues.New("fix me I'm wrong"),
+			err: clues.New("arbitrary error").
+				Label(graph.LabelStatus(http.StatusServiceUnavailable)),
 			opts: control.Options{
 				SkipTheseEventsOnInstance503: map[string][]string{
 					resourceID: {"bar", itemID},

src/pkg/services/m365/api/graph/errors.go

@@ -156,7 +156,8 @@ func stackWithCoreErr(ctx context.Context, err error, traceDepth int) error {
 		labels = append(labels, core.LabelRootCauseUnknown)
 	}
 
-	stacked := stackWithDepth(ctx, err, 1+traceDepth)
+	stacked := stackWithDepth(ctx, err, 1+traceDepth).
+		Label(LabelStatus(ode.Resp.StatusCode))
 
 	// labeling here because we want the context from stackWithDepth first
 	for _, label := range labels {