Improve readability of Windows connect error

Improve the readability of the connection error displayed to the user on
Windows when running docker commands fails by checking if the client is
privileged. If so then display the actual error wrapped in a generic
error "This error may indicate that the docker daemon is not running."

If not that display the actual error wrapped in a more specific error:
"In the default daemon configuration on Windows, the docker client must
be run with elevated privileges to connect."

Signed-off-by: Nick Adcock <nick.adcock@docker.com>
(cherry picked from commit 1a5dafb31eed7a44115f13595b54eaf109c4f388)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 239ac23799b7a15751d74616ef8357b40f3b759b
Component: engine

Author: zappy-shu

components/engine/client/request.go

@@ -178,7 +178,13 @@ func (cli *Client) doRequest(ctx context.Context, req *http.Request) (serverResp
 		// this is localised - for example in French the error would be
 		// `open //./pipe/docker_engine: Le fichier spécifié est introuvable.`
 		if strings.Contains(err.Error(), `open //./pipe/docker_engine`) {
-			err = errors.New(err.Error() + " In the default daemon configuration on Windows, the docker client must be run elevated to connect. This error may also indicate that the docker daemon is not running.")
+			// Checks if client is running with elevated privileges
+			if f, elevatedErr := os.Open("\\\\.\\PHYSICALDRIVE0"); elevatedErr == nil {
+				err = errors.Wrap(err, "In the default daemon configuration on Windows, the docker client must be run with elevated privileges to connect.")
+			} else {
+				f.Close()
+				err = errors.Wrap(err, "This error may indicate that the docker daemon is not running.")
+			}
 		}
 
 		return serverResp, errors.Wrap(err, "error during connect")