This particular contention goes right to the heart of the difference between Catholics and Protestants. This is why Luther was pressed on this very point. An erring Council means an erring Church, and an erring Church in matters of dogma flat-out falsifies what Christ himself promised. Is the Church full of sinners? Yes. Does the Church sin? No. When people sin gravely, they cut themselves off from the life-giving power of the Holy Spirit. He protects the Church, and most importantly, guides the Church into a deeper understanding of the deposit of faith she guards. I hope you realize that if this principle that (ecumenical) Councils may err were applied consistently, there is no reason to suppose that Nicea or Chalcedon is correct. That you do suppose them correct is an ad hoc appeal to Sacred Tradition. I hate it when that happens.