What did we learn from the Cowboys first preseason action? It wasn’t pretty, but preseason debuts rarely are. The Dallas Cowboys went up to Denver and put on a display that was, let’s say, less than flattering. It was riddled with mistakes that brought out so much laundry that the officials are going to have to do two separate loads. It’s not how you’d hope to see the team open the season, but that’s what they gave us. Here are five takeaways from this less-than-satisfying preseason opener. 1. They have a tackle problem Our worst fear is starting to come…