builder/vmware: Double check that OutputDirectory does not already exist

The initial check in Builder.Prepare does not use the OutputDir interface.
stepPrepareOutputDir also checks if OutputDir exists, error out there unless
using -force so we get the same behavior when RemoteType is esx5.

builder/vmware/step_prepare_output_dir.go

 package vmware
 
 import (
+	"fmt"
 	"github.com/mitchellh/multistep"
 	"github.com/mitchellh/packer/packer"
 	"log"
@@ -24,9 +25,14 @@ func (s *stepPrepareOutputDir) Run(state multistep.StateBag) multistep.StepActio
 		return multistep.ActionHalt
 	}
 
-	if exists && config.PackerForce {
-		ui.Say("Deleting previous output directory...")
-		dir.RemoveAll()
+	if exists {
+		if config.PackerForce {
+			ui.Say("Deleting previous output directory...")
+			dir.RemoveAll()
+		} else {
+			state.Put("error", fmt.Errorf("Output directory '%s' already exists.", config.OutputDir))
+			return multistep.ActionHalt
+		}
 	}
 
 	if err := dir.MkdirAll(); err != nil {