As far as I can surmise, Karl Rahner omitted this from the Denzinger because of his ecuмenistic tendencies. His theological works testify to this possibility. More than likely he saw the possibility of the Roman Catholic Church as never erring to be problematic to the Vatican II council which posits religious liberty, a reorientation of Protestants and other previously deemed heretics to be separated brethren, and the like. If the Roman Church can never err then what was already said cannot be changed and thus Vatican II becomes much more problematic.
Also on a side note, the Protestant Johann Gieseler in his Compendium of Ecclesiastical History, Vol I Pg. 434, mentions this condemned error of De Osma and says the following: "in other countries the same fate befel the opposite doctrines of the Gallican system."
So it seems that the idea of the Roman Church possibly falling into error was understood as Gallican even by Protestant historians, which was condemned by the Holy See on numerous occasions for its theological issues.
As far as "Ecclesia urbis Romae errare potest" means in Roman Catholic Doctrine is a much deeper subject. Most of the works dealing with are in Latin and hard to get a hold of, but I'll see if I can find anything that addresses the subject directly.