Cleaned up "docker insert".

Before, with Docker 0.6.2:

```
$ # Invalid Image ID.
$ sudo docker insert foo https://www.docker.io/static/img/docker-top-logo.png /foo

$ # Invalid URL.
$ docker insert 8dbd9 file:///foo /foo

$ # Success.
$ docker insert 8dbd9 https://www.docker.io/static/img/docker-top-logo.png /foo

5765ac1d7336c1d805f04e745b7fdc9583bb23202a825c6e26c933b636a357cd
5765ac1d7336: 

$ 
```

After:

```
$ # Invalid Image ID.
$ docker insert foo https://www.docker.io/static/img/docker-top-logo.png /foo
2013/09/20 05:01:46 Image does not exist: foo
$ # Invalid URL.
$ docker insert 8dbd9 file:///foo /foo
2013/09/20 05:02:39 Get file:///foo: unsupported protocol scheme "file"
$ # Success.
$ docker insert 8dbd9 https://www.docker.io/static/img/docker-top-logo.png /foo
c51a1d935c17: Image created
$
```

Fixes #1130.