Fix issue #1808

[pavdmyt]

Fixes #1808 by composing proper errors.NotFound object and raising it.

[pavdmyt]

@shin- I have updated the PR. Please take a look.

Now instead of catching requests.exceptions.ConnectionError it makes GET /containers/(id or name)/archive to check non-existent path case. According to Docker API the response object in success case (i.e. path exists inside container) contains HTTP 200 status code and X-Docker-Container-Path-Stat header is set. If path doesn't exist inside a container the response object simply has 404 status code and mentioned header is not set. I have found out that checking just for proper status code is enough.

Passing response object from GET /containers/(id or name)/archive to _raise_for_status method allows to reproduce the exact errors.NotFound instance as it would be produced in scenario when transferring a small file (when everything works fine).

I've also experimented with HEAD /containers/(id or name)/archive which could be a more "light-weight" implementation. The HEAD request has the same properties as GET in terms of status codes and X-Docker-Container-Path-Stat header behavior. However, passing the resulting response to _raise_for_status method doesn't allows to instantiate errors.NotFound instance with detalized error message like this:

docker.errors.NotFound: 404 Client Error: Not Found ("Could not find the file /foo in container ad1ca507a883de7dcea3a915bca9170cd6e1d3890fe1cec996618910f85781af")

Instead it returns this:

docker.errors.NotFound: 404 Client Error: Not Found

That's why I've decided to stick with GET.