如果用户ID或者密码无效则显示一条出错信息。
Show an error message if the user id or password is invalid.
如果不能验证数字签名，则运行时退出并给出错误信息。
If the digital signature cannot be verified, the runtime exits with an error.
通常，这只能在使用者输入数据后通过加以验证的方式来实现。如果数据无效，则弹出错误信息。
Typically, this is done by evaluating a user's entry after she has finished entering it, and throwing up an error message if it is invalid.
通常，这只能在使用者输入数据后通过加以验证的方式来实现。如果数据无效，则弹出错误信息。
Typically, this is done by evaluating a user's entry after she has finished entering it, and throwing up an error message if it is invalid.
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