Is it an error if any player in MLB drops a easy catch-able ball in foul area?

If the ball is in foul territory and the fielder does not catch the ball it is not a error. However for example lets say the ball was hit in foul territory down the first base line and the fielder caught the ball and there was a runner on 3rd base, If the runner decides to tag up and run home and the fielder made a wild throw, he would get an error for the throw.