Fix and improve Mongo commands

There's a bit more detail in the commit messages. I think the first two commits are uncontroversial, but I'd like to discuss the last one a bit as I'm aware it looks mental. But hey, Mongo.

(I asked about the [undocumented 252 exit code](http://docs.mongodb.org/v2.4/reference/exit-codes/) in their IRC channel and didn't receive a reply.)