Report an error if no flag(s) set in service update

For now if no flag(s) set, service update will still try to
do an update, it should return an error instead.

[issue 286](#286)

pkg/kn/commands/service/service_update.go

@@ -29,7 +29,7 @@ func NewServiceUpdateCommand(p *commands.KnParams) *cobra.Command {
 	var waitFlags commands.WaitFlags
 
 	serviceUpdateCommand := &cobra.Command{
-		Use:   "update NAME",
+		Use:   "update NAME [flags]",
 		Short: "Update a service.",
 		Example: `
   # Updates a service 'mysvc' with new environment variables
@@ -44,12 +44,20 @@ func NewServiceUpdateCommand(p *commands.KnParams) *cobra.Command {
 			if len(args) != 1 {
 				return errors.New("requires the service name.")
 			}
-
+			
 			namespace, err := p.GetNamespace(cmd)
 			if err != nil {
 				return err
 			}
 
+			if cmd.Flags().NFlag() == 0 {
+				return errors.New(fmt.Sprintf("Flag(s) not set\n See usage: %s", cmd.Use))
+			}
+
+			if cmd.Flags().NFlag() == 0 {
+				return fmt.Errorf("Flag(s) not set\nUsage: %s", cmd.Use)
+			}
+
 			client, err := p.NewClient(namespace)
 			if err != nil {
 				return err
@@ -91,10 +99,21 @@ func NewServiceUpdateCommand(p *commands.KnParams) *cobra.Command {
 				return nil
 			}
 		},
+		PreRunE: func(cmd *cobra.Command, args []string) error {
+			return preCheck(cmd, args)
+		},
 	}
 
 	commands.AddNamespaceFlags(serviceUpdateCommand.Flags(), false)
 	editFlags.AddUpdateFlags(serviceUpdateCommand)
 	waitFlags.AddConditionWaitFlags(serviceUpdateCommand, 60, "Update", "service")
 	return serviceUpdateCommand
 }
+
+func preCheck(cmd *cobra.Command, args []string) error {
+	if cmd.Flags().NFlag() == 0 {
+		return errors.New(fmt.Sprintf("flag(s) not set\nUsage: %s", cmd.Use))
+	}
+
+	return nil
+}

pkg/kn/commands/service/service_update_test.go

@@ -88,6 +88,26 @@ func fakeServiceUpdate(original *v1alpha1.Service, args []string, sync bool) (
 	return
 }
 
+func TestServcieUpdateNoFlags(t *testing.T) {
+	orig := newEmptyService()
+
+	action, _, _, err := fakeServiceUpdate(orig, []string{
+		"service", "update", "foo"}, false)
+
+	if action != nil {
+		t.Errorf("Unexpected action if no flag(s) set")
+	}
+
+	if err == nil {
+		t.Fatal(err)
+	}
+
+	expectedErrMsg := "flag(s) not set"
+	if !strings.Contains(err.Error(), expectedErrMsg) {
+		t.Fatalf("Missing %s in %s", expectedErrMsg, err.Error())
+	}
+}
+
 func TestServiceUpdateImageSync(t *testing.T) {
 	orig := newEmptyService()