Unauthorized push is not detected

Issue description

If no credentials have been provided during a push operation on a secured registry (like the official docker repository), docker-py call is successful (no docker.errors.APIError is thrown) but the error is reported in output messages.

Environment

This issue has been detected on Ubuntu 17.04 with the latest version of docker-py.

docker==2.5.1
docker-pycreds==0.2.1
Python 3.6.2 :: Anaconda, Inc.
Client:
 Version:      1.12.6
 API version:  1.24
 Go version:   go1.7.4
 Git commit:   78d1802
 Built:        Tue Mar 14 09:47:15 2017
 OS/Arch:      linux/amd64

Server:
 Version:      1.12.6
 API version:  1.24
 Go version:   go1.7.4
 Git commit:   78d1802
 Built:        Tue Mar 14 09:47:15 2017
 OS/Arch:      linux/amd64

Code snippet

import docker
cli = docker.from_env(version='auto')
res = cli.images.push('centos')  # No error thrown here...
print(res)
# {"status":"The push refers to a repository [docker.io/library/centos]"}
# {"status":"Preparing","progressDetail":{},"id":"cf516324493c"}
# {"errorDetail":{"message":"unauthorized: authentication required"},"error":"unauthorized: authentication required"}

The reproducer enters through cli.images.push('centos'); start by tracing the push entry point and how response messages are handled. Done means an unauthorized response is surfaced as docker.errors.APIError instead of only appearing in the returned output.