ref(http.go): improve client error message

This PR improves the resultant client error message they get when the returned response is
incorrect. This is especially important for determining the error message

Old message when using a v2 client with a v1 cluster:

```
http://deis.10.245.1.3.xip.io does not appear to be a valid Deis controller. Make sure that the
Controller URI is correct and the server is running.
```

New message:

```
http://deis.10.245.1.3.xip.io/v2/ does not appear to be a valid Deis controller. Make sure that the
Controller URI is correct and the server is running.
```

Notice the appended /v2/ path, which users can assume this is a v2 client.

Author: mboersma

client/http.go

@@ -156,9 +156,8 @@ func checkForErrors(res *http.Response, body string) error {
 // CheckConnection checks that the user is connected to a network and the URL points to a valid controller.
 func CheckConnection(client *http.Client, controllerURL url.URL) error {
 	errorMessage := `%s does not appear to be a valid Deis controller.
-Make sure that the Controller URI is correct and the server is running.`
-
-	baseURL := controllerURL.String()
+Make sure that the Controller URI is correct, the server is running and
+your client version is correct.`
 
 	controllerURL.Path = "/v2/"
 
@@ -172,13 +171,13 @@ Make sure that the Controller URI is correct and the server is running.`
 	res, err := client.Do(req)
 
 	if err != nil {
-		fmt.Printf(errorMessage+"\n", baseURL)
+		fmt.Printf(errorMessage+"\n", controllerURL.String())
 		return err
 	}
 	defer res.Body.Close()
 
 	if res.StatusCode != 401 {
-		return fmt.Errorf(errorMessage, baseURL)
+		return fmt.Errorf(errorMessage, controllerURL.String())
 	}
 
 	checkAPICompatibility(res.Header.Get("DEIS_API_VERSION"))