tuned error -- reduce potential for confusion in a higher-level context, e.g. partial checking of theory sub-graph;

--- a/src/Pure/Isar/toplevel.ML	Fri May 11 13:41:30 2012 +0200
+++ b/src/Pure/Isar/toplevel.ML	Thu May 17 15:23:00 2012 +0200
@@ -188,7 +188,7 @@
   | _ => raise UNDEF);
 
 fun end_theory _ (State (NONE, SOME (Theory (Context.Theory thy, _)))) = thy
-  | end_theory pos (State (NONE, _)) = error ("Missing theory" ^ Position.str_of pos)
+  | end_theory pos (State (NONE, _)) = error ("Bad theory" ^ Position.str_of pos)
   | end_theory pos (State (SOME _, _)) = error ("Unfinished theory" ^ Position.str_of pos);