builder/virtualbox: error if guest additions URL can't be read [GH-1439]

CHANGELOG.md

@@ -34,6 +34,8 @@ BUG FIXES:
   * builder/qemu: If headless, sdl display won't be used. [GH-1395]
   * builder/qemu: Use `512M` as `-m` default. [GH-1444]
   * builder/virtualbox/all: Seed RNG to avoid same ports. [GH-1386]
+  * builder/virtualbox/all: Better error if guest additions URL couldn't be
+      detected. [GH-1439]
   * builder/vmware/all: `ssh_host` accepts templates. [GH-1396]
   * builder/vmware/all: Don't remount floppy in VMX post step. [GH-1239]
   * builder/vmware/vmx: Do not re-add floppy disk files to VMX [GH-1361]

builder/virtualbox/common/step_download_guest_additions.go

@@ -91,6 +91,13 @@ func (s *StepDownloadGuestAdditions) Run(state multistep.StateBag) multistep.Ste
 				additionsName)
 		}
 	}
+	if url == "" {
+		err := fmt.Errorf("Couldn't detect guest additions URL.\n" +
+			"Please specify `guest_additions_url` manually.")
+		state.Put("error", err)
+		ui.Error(err.Error())
+		return multi.ActionHalt
+	}
 
 	if checksumType != "none" {
 		if s.GuestAdditionsSHA256 != "" {