v12.x is actually VS2013, see this list.
Though it's odd that it wants to run "/Upgrade". Can't remember this happening for me. It looks like the project upgrader is triggered by the cocos command which causes cocos to abort compilation because it takes the conversion process' return code 1 as a failure code. That's just an assumption but may be worth investigating. Found mentions of the /Upgrade issue in this post and this post.
You may want to try VS2015 Community. It's free and from what I heard not that different from Pro. Though I'm not really aware of any restrictions, it was simply my first pick due to its unbeatable price point.