The code to present or "throw" a custom error is fairly simple. You start off by making a new instance "instantiating" the Error Class which is part of the standard AS3 library.

This class takes a string as an argument, that is displayed to the debug box and screen if in debug mode.

throw new Error("Sorry, I just have to present this great error now!");

Note: Note, this will create a fatal error and will display a dialog at runtime if the debug player is used.

Thats all there is to creating a custom error