Use constant error for Sessions

melody.go

@@ -277,7 +277,7 @@ func (m *Melody) BroadcastBinaryOthers(msg []byte, s *Session) error {
 // Sessions returns all sessions. An error is returned if the melody session is closed.
 func (m *Melody) Sessions() ([]*Session, error) {
 	if m.hub.closed() {
-		return nil, errors.New("melody instance is closed")
+		return nil, ErrClosed
 	}
 	return m.hub.all(), nil
 }