chainntnfs: declare common ErrChainNotifierShuttingDown for all backends

chainntnfs/interface.go

@@ -1,6 +1,7 @@
 package chainntnfs
 
 import (
+	"errors"
 	"fmt"
 	"sync"
 
@@ -9,6 +10,12 @@ import (
 	"github.com/btcsuite/btcd/wire"
 )
 
+var (
+	// ErrChainNotifierShuttingDown is used when we are trying to
+	// measure a spend notification when notifier is already stopped.
+	ErrChainNotifierShuttingDown = errors.New("chain notifier shutting down")
+)
+
 // TxConfStatus denotes the status of a transaction's lookup.
 type TxConfStatus uint8