Fix error counter - fixes #3650

For few commands, RClone counts a error multiple times. This was fixed by
creating a new error type which keeps a flag to remember if the error has
already been counted or not. The CountError function now wraps the original
error eith the above new error type and returns it.

fs/sync/sync.go

@@ -926,7 +926,7 @@ func MoveDir(ctx context.Context, fdst, fsrc fs.Fs, deleteEmptySrcDirs bool, cop
 			fs.Infof(fdst, "Server side directory move succeeded")
 			return nil
 		default:
-			fs.CountError(err)
+			err = fs.CountError(err)
 			fs.Errorf(fdst, "Server side directory move failed: %v", err)
 			return err
 		}