return error message from using --untag with nonexistent tag

pkg/kn/commands/service/update_test.go

@@ -737,6 +737,15 @@ func TestServiceUpdateDeletionTimestampNotNil(t *testing.T) {
 	assert.ErrorContains(t, err, "service")
 }
 
+func TestServiceUpdateTagDoesNotExist(t *testing.T) {
+	orig := newEmptyService()
+
+	_, _, _, err := fakeServiceUpdate(orig, []string{
+		"service", "update", "foo", "--untag", "foo", "--no-wait"})
+
+	assert.Error(t, err, "Error: tag foo does not exist")
+}
+
 func newEmptyService() *servingv1.Service {
 	ret := &servingv1.Service{
 		TypeMeta: metav1.TypeMeta{

pkg/kn/traffic/compute.go

@@ -83,13 +83,15 @@ func (e ServiceTraffic) isTagPresent(tag string) bool {
 	return false
 }
 
-func (e ServiceTraffic) untagRevision(tag string) {
+func (e ServiceTraffic) untagRevision(tag string) error {
 	for i, target := range e {
 		if target.Tag == tag {
 			e[i].Tag = ""
-			break
+			return nil
 		}
 	}
+
+	return fmt.Errorf("Error: tag %s does not exist", tag)
 }
 
 func (e ServiceTraffic) isRevisionPresent(revision string) bool {
@@ -276,8 +278,17 @@ func Compute(cmd *cobra.Command, targets []servingv1.TrafficTarget, trafficFlags
 	traffic := newServiceTraffic(targets)
 
 	// First precedence: Untag revisions
+	var errStrings []string
 	for _, tag := range trafficFlags.UntagRevisions {
-		traffic.untagRevision(tag)
+		err = traffic.untagRevision(tag)
+		if err != nil {
+			errStrings = append(errStrings, err.Error())
+		}
+	}
+
+	// Return all errors from untagging revisions
+	if len(errStrings) > 0 {
+		return nil, fmt.Errorf(strings.Join(errStrings, "\n"))
 	}
 
 	for _, each := range trafficFlags.RevisionsTags {

pkg/kn/traffic/compute_test.go

@@ -278,6 +278,18 @@ func TestComputeErrMsg(t *testing.T) {
 			[]string{"--traffic", "echo-v1=40", "--traffic", "echo-v1=60"},
 			"repetition of revision reference echo-v1 is not allowed, use only once with --traffic flag",
 		},
+		{
+			"untag single tag that does not exist",
+			append(newServiceTraffic([]servingv1.TrafficTarget{}), newTarget("latest", "echo-v1", 100, false)),
+			[]string{"--untag", "foo"},
+			"Error: tag foo does not exist",
+		},
+		{
+			"untag multiple tags that do not exist",
+			append(newServiceTraffic([]servingv1.TrafficTarget{}), newTarget("latest", "echo-v1", 100, false)),
+			[]string{"--untag", "foo", "--untag", "bar"},
+			"Error: tag foo does not exist\nError: tag bar does not exist",
+		},
 	} {
 		t.Run(testCase.name, func(t *testing.T) {
 			testCmd, tFlags := newTestTrafficCommand()