Don't create Artifact if there were errors

Prevents nil-dereference crash when errors occur

builder/xenserver/builder.go

@@ -361,12 +361,20 @@ func (self *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (pa
 	self.runner = &multistep.BasicRunner{Steps: steps}
 	self.runner.Run(state)
 
-	artifact, _ := NewArtifact(self.config.OutputDir)
-
 	if rawErr, ok := state.GetOk("error"); ok {
 		return nil, rawErr.(error)
 	}
 
+	// If we were interrupted or cancelled, then just exit.
+	if _, ok := state.GetOk(multistep.StateCancelled); ok {
+		return nil, errors.New("Build was cancelled.")
+	}
+	if _, ok := state.GetOk(multistep.StateHalted); ok {
+		return nil, errors.New("Build was halted.")
+	}
+
+	artifact, _ := NewArtifact(self.config.OutputDir)
+
 	return artifact, nil
 }