Fall back to errmsg when creating MongoError for command errors

Hi Christian,

When commands like geoHaystack fail against 2.6, you get back really unhelpful "MongoError: n/a" errors, when errmsg contains much more useful info. Not sure if this is something you did intentionally, but it'll knock out at least one of the handful of remaining issues for [upgrading mongoose to driver 2.x](https://github.com/LearnBoost/mongoose/compare/driver-upgrade)