By Zach Koons Liverpool’s Sadio Mané cashed in on the goalkeeper’s mistake. View the original article to see embedded media. A slow start in the FA Cup semifinals against Liverpool only grew worse for Manchester City when goalkeeper Zack Steffen yielded an embarrassing goal in the first half. Already trailing 1–0 in the 17th minute, the Manchester City defense passed the ball back to Steffen. The keeper fielded it cleanly, but then attempted to take a dribble, allowing Liverpool forward Sadio Mané to slide in and deflect the ball into the net. Liverpool took a two-nil advantage due to Steffen’…