You mix up the uspconfig server and the ispconfig interface. What you wrote above is a ispconfig server plugin, server plugins are used to configure services like apache, postfix etc. When you login to ispconfig, then you interact with the ispconfig interface and not the server, so if you want to react on failed logins, you will have tp write code for the interface and not the server. So your plugin can not work form a logical standpoint and it will never catch a interface event on the server.
If you want to log actions of the interface, take a look at the ispconfig svn to see how the login function that you want to replicate is implemented there.