wal: log errors in wal.Close()

This patch adds error logging in wal.Close() if unlocking and
destroying fail. Though it is hard to handling the errors, logging
would be helpful for trouble shooting.

wal/wal.go

@@ -449,9 +449,14 @@ func (w *WAL) Close() error {
 		}
 	}
 	for _, l := range w.locks {
-		// TODO: log the error
-		l.Unlock()
-		l.Destroy()
+		err := l.Unlock()
+		if err != nil {
+			plog.Errorf("failed to unlock during closing wal: %s", err)
+		}
+		err = l.Destroy()
+		if err != nil {
+			plog.Errorf("failed to destroy lock during closing wal: %s", err)
+		}
 	}
 	return nil
 }