fixes the now-incorrect module for ApiError -- noticed since 0.3.2

https://github.com/dotcloud/docker-py/blob/0.3.2/docker/errors.py
- ApiError is now(0.3.2) under the docker.errors

this is the commit that made it break: https://github.com/dotcloud/docker-py/commit/18d4db09ecc79cef49830fa2c28c5e99dd02be42