Capturing Uncaught Exceptions
One important note on XPages custom error pages. Never use XPages events on an error page. For example, a Section control on an XPage adds events to expand / collapse the section. The section will work as normal if the error page is triggered from a full refresh. But if the error page is triggered from a partial refresh, the XSP Command Manager only prints HTML to the browser, it doesn't attach events. So your section will not work. Error is the custom error page in this application and gives an example of an XPage with only standard HTML.
NOTE: The project requires the XPages Extension Library to be installed on the server.
M2 - M5 included versions that did not require the XPages Extension Library. However, I do not recommend XPages development without using the XPages Extension Library - it is a significantly weaker subset of functionality.