ignore system messages;

--- a/src/Pure/System/session.scala	Sat May 22 20:20:51 2010 +0200
+++ b/src/Pure/System/session.scala	Sat May 22 20:37:20 2010 +0200
@@ -148,7 +148,7 @@
       }
       else if (result.kind == Isabelle_Process.Kind.EXIT)
         prover = null
-      else if (result.kind != Isabelle_Process.Kind.STDIN && !result.is_raw)
+      else if (!result.is_system)   // FIXME syslog (!?)
         bad_result(result)
     }