clarified exit sequence: prover is reset afterwards, no more output messages;

--- a/src/Pure/System/isabelle_process.scala	Tue Apr 29 21:11:24 2014 +0200
+++ b/src/Pure/System/isabelle_process.scala	Tue Apr 29 21:29:36 2014 +0200
@@ -97,10 +97,9 @@
 
     process.stdin.close
     if (startup_failed) {
-      exit_message(127)
-      Thread.sleep(300)
       terminate_process()
       process_result.join
+      exit_message(127)
     }
     else {
       val (command_stream, message_stream) = system_channel.rendezvous()