Bug 964342: Send language setting to basket.

This seems like the way it should work, but correct me if I'm wrong. I tried to ensure that the language setting wasn't required for the subscription call to happen. I'll see what I can do about adding some tests.

I also went ahead and added the [basket error codes](https://github.com/mozilla/basket-client/blob/master/basket/errors.py) to basket-client.js and used the proper code instead of searching through the error description. If this needs to be split into a separate PR let me know.

@fcampo r?