Emacs Lisp style on callbacks?

[Elena]

Hello,

this a question about style.

I'm writing an asynchronous function which upon completion will
execute a callback.  Would you suggest using separate callbacks for
successful and unsuccessful completion? or would you suggest using the
same callback which takes a cons with a success flag as its car?  Any
other suggestions?

I know Emacs Lisp lacks closures: the function would take care of all
additional book-keeping.

Thank you.