How much have Marcus Semien's errors actually cost the Oakland A's?

Kelley L Cox-USA TODAY SportsI'm not a big fan of measuring defense by errors because I think the stat is inherently flawed. It's missing too much context, and it's a loose judgment call that can be subject to the whims of different scorers. Citing how many errors a player has made, especially an outfielder, tells me next to nothing about his defensive ability. However, there are extremes to any example, and at some point when a shortstop like Marcus Semien makes 15 errors in 40* games there is no question that he has been brutal with the glove. In this case, the errors are sending a clear message.

What do 15 errors really mean, though? What kind of effect does that have on a team, and how…