When we had to return errors about not finding assertions in the assertions service we introduced store.AssertionNotFound so to carry a bit more information and have more informative messages.
Over time it is clear that having parallel asserts.ErrNotFound and store.AssertionNotFoundError is a nuisance, sometimes confusing, sometimes requiring converting back and forth.
Plan is to just have a slightly tweaked asserts.AssertionNotFoundError and a asserts.IsNotFound(err) helper for checks.