fixed Isabelle/Scala build (cf. f3220ef79d51);

--- a/src/Pure/System/session.scala	Sat Aug 14 23:01:53 2010 +0200
+++ b/src/Pure/System/session.scala	Sun Aug 15 13:17:45 2010 +0200
@@ -318,7 +318,7 @@
           state_snapshot.lookup_command(id)
         def state(command: Command): Command.State =
           try { state_snapshot.command_state(document, command) }
-          catch { case _: State.Fail => command.empty_state }
+          catch { case _: Document.State.Fail => command.empty_state }
       }
     }