internal/getproviders: Add URL to error message for clarity

Occasionally `terraform init` on some providers may return the following error message:

Error while installing citrix/citrixadc v1.13.0: could not query provider
registry for registry.terraform.io/citrix/citrixadc: failed to retrieve
authentication checksums for provider: 403 Forbidden

The 403 is most often returned from GitHub (rather than Registry API)
and this change makes it more obvious.

internal/getproviders/registry_client.go

@@ -437,7 +437,7 @@ func (c *registryClient) getFile(url *url.URL) ([]byte, error) {
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("%s", resp.Status)
+		return nil, fmt.Errorf("%s returned from %s", resp.Status, resp.Request.URL)
 	}
 
 	data, err := ioutil.ReadAll(resp.Body)