premature window close means failure;

--- a/src/Pure/Tools/build_dialog.scala	Sat Jan 05 18:42:29 2013 +0100
+++ b/src/Pure/Tools/build_dialog.scala	Sat Jan 05 19:05:16 2013 +0100
@@ -67,7 +67,7 @@
     /* GUI state */
 
     private var is_stopped = false
-    private var return_code = 0
+    private var return_code = 2
 
     override def closeOperation { sys.exit(return_code) }