根据学生输入的成绩判断学生的等级！添加了输入控制，当输入不正确时提示出错（例如输入字符，或输入的分数大于100小于0）。-students admitted under the judgment results of grading students! Add the importation of control when the input is not correct mistakes suggested (for example, the importation of characters, or the importation of more than 100 points less than 0).