tuned -- potentially more robust;

--- a/src/Pure/PIDE/session.scala	Fri Apr 25 23:47:39 2014 +0200
+++ b/src/Pure/PIDE/session.scala	Sat Apr 26 00:20:26 2014 +0200
@@ -464,9 +464,9 @@
               phase = Session.Ready
 
             case Markup.Return_Code(rc) if output.is_exit =>
-              prover = None
               if (rc == 0) phase = Session.Inactive
               else phase = Session.Failed
+              prover = None
 
             case _ => raw_output_messages.post(output)
           }