builder/virtualbox: Don't validate guest additions URL so early because it requires template

builder/virtualbox/builder.go

@@ -226,14 +226,6 @@ func (b *Builder) Prepare(raws ...interface{}) error {
 		b.config.GuestAdditionsSHA256 = strings.ToLower(b.config.GuestAdditionsSHA256)
 	}
 
-	if b.config.GuestAdditionsURL != "" {
-		b.config.GuestAdditionsURL, err = common.DownloadableURL(b.config.GuestAdditionsURL)
-		if err != nil {
-			errs = packer.MultiErrorAppend(
-				errs, fmt.Errorf("guest_additions_url: %s", err))
-		}
-	}
-
 	if !b.config.PackerForce {
 		if _, err := os.Stat(b.config.OutputDir); err == nil {
 			errs = packer.MultiErrorAppend(

builder/virtualbox/step_download_guest_additions.go

@@ -92,6 +92,14 @@ func (s *stepDownloadGuestAdditions) Run(state map[string]interface{}) multistep
 			additionsName)
 	}
 
+	url, err = common.DownloadableURL(url)
+	if err != nil {
+		err := fmt.Errorf("Error preparing guest additions url: %s", err)
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
 	log.Printf("Guest additions URL: %s", url)
 
 	log.Printf("Acquiring lock to download the guest additions ISO.")