How to test pages for HTTP status 403, 404, 500, 503

[albertkao]

I developed a web application with Java (Struts 2) and Web Experience Toolkit (WET - https://github.com/wet-boew/wet-boew), which is running on the WebLogic 12c server.
When any one of the HTTP status codes 403, 404, 500, 503 occurs, my web application will display a corresponding web page.
e.g. When the HTTP status code 503 occurs, my web application will display the web page 503.html.
How to set up or simulate the conditions to test my web application web pages for HTTP status codes 403, 404, 500, 503?
i.e. simulate the HTTP status codes 503, etc so as to test my web application UI to ensure that 503 is handled properly, assuming that the webserver works properly.