Improve message when pushing a non-existent image

I was confused earlier when I did:

```
docker push localhost.localdomain:1234/foo
```

Because docker told me:

```
No such id: localhost.localdomain:1234/foo
```

I actually had buried in my mind the solution to this, but the error message
confused me because I had recently had some fun trying to get the registry
working and therefore thought it was telling me that I didn't have an account
on the registry.

This pull request makes it unambiguous that the error is that the specified
image is unknown.

/cc @cpuguy83