vss: Change ErrorHandler signature

We don't need `error` here: the only existing implementation
of `ErrorHandler` always call `Backup.Error` and all
implementations of `Backup.Error` always return nil.

cmd/restic/cmd_backup.go

@@ -632,8 +632,8 @@ func runBackup(opts BackupOptions, gopts GlobalOptions, term *termstatus.Termina
 			return err
 		}
 
-		errorHandler := func(item string, err error) error {
-			return p.Error(item, nil, err)
+		errorHandler := func(item string, err error) {
+			p.Error(item, nil, err)
 		}
 
 		messageHandler := func(msg string, args ...interface{}) {

internal/fs/fs_local_vss.go

@@ -43,8 +43,8 @@ func ParseVSSConfig(o options.Options) (VSSConfig, error) {
 	return cfg, nil
 }
 
-// ErrorHandler is used to report errors via callback
-type ErrorHandler func(item string, err error) error
+// ErrorHandler is used to report errors via callback.
+type ErrorHandler func(item string, err error)
 
 // MessageHandler is used to report errors/messages via callbacks.
 type MessageHandler func(msg string, args ...interface{})

internal/fs/fs_local_vss_test.go

@@ -88,10 +88,8 @@ func TestVSSConfig(t *testing.T) {
 				t.Fatal(err)
 			}
 
-			errorHandler := func(item string, err error) error {
+			errorHandler := func(item string, err error) {
 				t.Fatalf("unexpected error (%v)", err)
-
-				return nil
 			}
 			messageHandler := func(msg string, args ...interface{}) {
 				t.Fatalf("unexpected message (%s)", fmt.Sprintf(msg, args))
@@ -181,10 +179,8 @@ func TestParseMountPoints(t *testing.T) {
 			}
 
 			var log []string
-			errorHandler := func(item string, err error) error {
+			errorHandler := func(item string, err error) {
 				log = append(log, strings.TrimSpace(err.Error()))
-
-				return nil
 			}
 			messageHandler := func(msg string, args ...interface{}) {
 				t.Fatalf("unexpected message (%s)", fmt.Sprintf(msg, args))