Invocation of the front-end error logger

The logger subscribes to the global JS event “onerror”. This logs all unexpected exceptions. It also implements the Ext.Error.handle method to receive all defined errors in the application. This implies the usage of the Ext.Error.raise method. Since this method is actually not used consistently, we have to review and refactor some code in the frontend. One example is the ServerException::handleFailedRequest method, there we can track AJAX connection problems.
A DOS and “No Connection” Protection is needed. That means if the same error is raised multiple times in short time interval, the error should be logged only once. If the logging could not be saved multiple times, this means there is no connection to the server. This error should be shown to the user.