When Mariners President Kevin Mather on Friday characterized being a baseball general manager as “an impossible job,” he was speaking about the complexities and vast scope of the position. But for the past decade, that description has been all too true in Seattle. It’s been an impossible job for the sitting Mariners GM to do well, at least, and Friday, Jack Zduriencik’s failures, like that of his predecessor, Bill Bavasi, got him fired.