fix(BranchBarrier): fix QueryPrepared method not return real insert exception

[wooln]

QueryPrepared调用的DbUtils.InsertBarrier里面有try了，返回插入行数+异常的元组，当插入遇到意料外的异常（如账号权现不能写），之前它还会往下走，导致reason.Equals出现null引用，虽然最终也是异常，但尽量返回真正的异常。

参考了go client的实现：

// QueryPrepared queries prepared data
func (bb *BranchBarrier) QueryPrepared(db *sql.DB) error {
	_, err := dtmimp.InsertBarrier(db, bb.TransType, bb.Gid, dtmimp.MsgDoBranch0, dtmimp.MsgDoOp, dtmimp.MsgDoBarrier1, dtmimp.OpRollback, bb.DBType, bb.BarrierTableName)
	var reason string
	if err == nil {
		sql := fmt.Sprintf("select reason from %s where gid=? and branch_id=? and op=? and barrier_id=?", dtmimp.BarrierTableName)
		sql = dtmimp.GetDBSpecial(bb.DBType).GetPlaceHoldSQL(sql)
		logger.Debugf("queryrow: %s", sql, bb.Gid, dtmimp.MsgDoBranch0, dtmimp.MsgDoOp, dtmimp.MsgDoBarrier1)
		err = db.QueryRow(sql, bb.Gid, dtmimp.MsgDoBranch0, dtmimp.MsgDoOp, dtmimp.MsgDoBarrier1).Scan(&reason)
	}
	if reason == dtmimp.OpRollback {
		return ErrFailure
	}
	return err
}