Return Error info from ActiveX Dll to User Application

[asbo]

I create ActiveX Dll to serve my own applications.
What is the best way to return error from dll to app?

My dll has the Dll_class and several support modules.
And app has the App_class, which create the instance of Dll_Class with events.
In App_module declared and created several instances of App_class for various purposes.

Dll_class functions returns any values on success and 0(zero) if something goes wrong.
However, cause this 0(zero) is noninformative, so I create in Dll_class a special function,
that returns my own last error number and description, when App recieving 0(zero) call its.
This Error values saved in class level variables and refreshed each time, when Dll_class function decide to return 0(zero).

Maybe I invent an another bicycle, but there are best practices solutions for a long time already?..

Solutions are existing of course, but, unfortunately, not in those places where I was intensively looking for it :)
.