Well every goal is someone's fault but for the first it was very similar to Edison last weekend. Edison got away with it because Jota wasn't as awake (too late closing down) but Mane was right on Steffen. For his second I don't think any other keeper would have expected him to hit it on the volley near post (and when he hit it Steffen's feet were off the ground as he was moving to his right) so it's a little bit of a disservice to blame the keeper, IMO.
Mane also kept possession a lot better in this game (I've been highly critical of him this season in this regard) and brought other players into the game. Mo should have scored from his excellent pass too. I've often been critical of Mane this season as I think he's declined, however today, and through the middle, he really stepped up.