After understanding all the logic and workings of the Handler Exception, which are in the package: src/main/java/reactor/arc/error.
Create a new Exception flow for NotFound.
You'll need to create the handleNotFoundException method in WebErrorHandler.java and create your own Exception from the pattern of the other exceptions in the directory: src/main/java/reactor/arc/error/exception.
After understanding all the logic and workings of the Handler Exception, which are in the package: src/main/java/reactor/arc/error. Create a new Exception flow for NotFound. You'll need to create the handleNotFoundException method in WebErrorHandler.java and create your own Exception from the pattern of the other exceptions in the directory: src/main/java/reactor/arc/error/exception.