In English, 'adjective noun' does not necessarily imply the existence of 'not-adjective' nouns, and the adjective may serve as a reminder or reason.  For instance, "This job is too dangerous for mortal humans!"

In the current context, failure of 'iter(ob)', by itself, only tells us that the particular object ob is not iterable. "'X' object is not iterable", rather that "'X' objects are not iterable", is correct.  (Ditto for other messages.)

That said, I think "cannot unpack int object, because not iterable" is better.