channelnotifier+discover+invoices: return error in Stop functions

In order to be consistent with other sub systems an error is now
returned from the Stop functions.
This also allows writing a generic cleanup mechanism to stop all
sub systems in case of a failure.

invoices/invoiceregistry.go

@@ -237,7 +237,7 @@ func (i *InvoiceRegistry) Start() error {
 	// delete them.
 	err = i.scanInvoicesOnStart()
 	if err != nil {
-		i.Stop()
+		_ = i.Stop()
 		return err
 	}
 
@@ -245,12 +245,13 @@ func (i *InvoiceRegistry) Start() error {
 }
 
 // Stop signals the registry for a graceful shutdown.
-func (i *InvoiceRegistry) Stop() {
+func (i *InvoiceRegistry) Stop() error {
 	i.expiryWatcher.Stop()
 
 	close(i.quit)
 
 	i.wg.Wait()
+	return nil
 }
 
 // invoiceEvent represents a new event that has modified on invoice on disk.

invoices/invoiceregistry_test.go

@@ -527,7 +527,11 @@ func TestCancelHoldInvoice(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer registry.Stop()
+	defer func() {
+		if err := registry.Stop(); err != nil {
+			t.Fatalf("failed to stop invoice registry: %v", err)
+		}
+	}()
 
 	// Add the invoice.
 	_, err = registry.AddInvoice(testHodlInvoice, testInvoicePaymentHash)
@@ -1005,7 +1009,9 @@ func TestInvoiceExpiryWithRegistry(t *testing.T) {
 
 	// Give some time to the watcher to cancel everything.
 	time.Sleep(500 * time.Millisecond)
-	registry.Stop()
+	if err = registry.Stop(); err != nil {
+		t.Fatalf("failed to stop invoice registry: %v", err)
+	}
 
 	// Create the expected cancellation set before the final check.
 	expectedCancellations = append(

invoices/test_utils_test.go

@@ -215,7 +215,9 @@ func newTestContext(t *testing.T) *testContext {
 		clock:    clock,
 		t:        t,
 		cleanup: func() {
-			registry.Stop()
+			if err = registry.Stop(); err != nil {
+				t.Fatalf("failed to stop invoice registry: %v", err)
+			}
 			cleanup()
 		},
 	}