Improve readability of ISO download errors + include URL.

This is a separate issue I bumped into while developing the dashboard
PR.

pkg/minikube/cluster/cluster.go

@@ -217,7 +217,7 @@ func createHost(api libmachine.API, config cfg.MachineConfig) (*host.Host, error
 
 	if config.VMDriver != "none" {
 		if err := config.Downloader.CacheMinikubeISOFromURL(config.MinikubeISO); err != nil {
-			return nil, errors.Wrap(err, "Error attempting to cache minikube ISO from URL")
+			return nil, errors.Wrap(err, "unable to cache ISO")
 		}
 	}
 

pkg/util/downloader.go

@@ -72,9 +72,9 @@ func (f DefaultDownloader) CacheMinikubeISOFromURL(isoURL string) error {
 		options.ChecksumHash = crypto.SHA256
 	}
 
-	fmt.Println("Downloading Minikube ISO")
+	fmt.Printf("Downloading Minikube ISO: %s\n", isoURL)
 	if err := download.ToFile(isoURL, f.GetISOCacheFilepath(isoURL), options); err != nil {
-		return errors.Wrap(err, "Error downloading Minikube ISO")
+		return errors.Wrap(err, "ToFile")
 	}
 
 	return nil

pkg/util/utils.go

@@ -97,14 +97,17 @@ func Retry(attempts int, callback func() error) (err error) {
 func RetryAfter(attempts int, callback func() error, d time.Duration) (err error) {
 	m := MultiError{}
 	for i := 0; i < attempts; i++ {
+		glog.V(1).Infof("retry loop %d", i)
 		err = callback()
 		if err == nil {
 			return nil
 		}
 		m.Collect(err)
 		if _, ok := err.(*RetriableError); !ok {
+			glog.Infof("non-retriable error: %v", err)
 			return m.ToError()
 		}
+		glog.V(2).Infof("sleeping %s", d)
 		time.Sleep(d)
 	}
 	return m.ToError()