Added specific error message when hitting 401 over HTTP on push

This clarifies the situation for people hitting the issue outline in #4343. Instead of

```sh
$ docker push my.registry.io/ubuntu
The push refers to a repository [my.registry.io/ubuntu] (len: 1)
Sending image list
Pushing repository my.registry.io/ubuntu (1 tags)
511136ea3c5a: Pushing 
2014/04/08 14:55:57 HTTP code 401 while uploading metadata: invalid character '<' looking for beginning of value
```

People would now see 
```sh
$ docker push my.registry.io/ubuntu
The push refers to a repository [my.registry.io/ubuntu] (len: 1)
Sending image list
Pushing repository my.registry.io/ubuntu (1 tags)
511136ea3c5a: Pushing 
2014/04/08 16:40:27 HTTP code 401, Docker will not send auth headers over HTTP.
```

More details on this issue: dotcloud/docker-registry#298