ignore spurious TIMEOUT messages, maybe caused by change of actor semantics in scala-2.8;

--- a/src/Pure/System/session.scala	Mon May 10 15:00:11 2010 +0200
+++ b/src/Pure/System/session.scala	Mon May 10 15:01:25 2010 +0200
@@ -227,6 +227,8 @@
         case result: Isabelle_Process.Result =>
           handle_result(result.cache(xml_cache))
 
+        case TIMEOUT =>  // FIXME clarify!
+
         case bad if prover != null =>
           System.err.println("session_actor: ignoring bad message " + bad)
       }