stop website from loading after a catch and show error. Inside a class

Question

Hi, I am having a problem with a Try and Catch in a Web Application, pretty much the problem is: I have my function to open a port, if the reader is not connected then it will catch it but I am not sure on the following: Showing the error in a label (when working from inside a class) and how to stop the WebApp from finishing loading so it doesn't load the rest of the code. I can't use Java because of Users Requirements so this is why I am a bit stuck as MessageBox doesn't exist anymore. Many Thanks