OP-5083 - Fix error code return for an invalid coupon.

If the coupon cannot be found, a 404 is thrown.

Updating the documentation to reflect that.

![screen shot 2014-03-25 at 10 59 05 am](https://f.cloud.github.com/assets/358891/2515820/3df7952c-b447-11e3-83fb-2702f3a71e17.png)