解析json失败，想要捕获异常，可以使用pacll

      local str = '[{"name":"fjd","age":110},{"date":"2017-04-25","age":111,"name":"wyc"}]'
      local err,tbl = pcall(function(str)return cjson.decode(str)end,str)

正常err返回true，tbl是正常的结果。错误err返回FALSE，tbl返回错误信息，如Expected colon but found invalid token at character 10