avoid repeated calls to metis from "resolve_tac" in case of ultimate failure

--- a/src/HOL/Tools/Metis/metis_tactic.ML	Thu Jan 03 00:02:15 2013 +0100
+++ b/src/HOL/Tools/Metis/metis_tactic.ML	Thu Jan 03 09:56:39 2013 +0100
@@ -218,13 +218,10 @@
                   | _ => (trace_msg ctxt (fn () => "Metis: No result"); [])
             end
         | Metis_Resolution.Satisfiable _ =>
-            (trace_msg ctxt (fn () => "Metis: No first-order proof with the lemmas supplied");
-             if null fallback_type_encs then
-               ()
-             else
-               raise METIS ("FOL_SOLVE",
-                            "No first-order proof with the lemmas supplied");
-             [])
+            (trace_msg ctxt (fn () =>
+               "Metis: No first-order proof with the lemmas supplied");
+             raise METIS ("FOL_SOLVE",
+                          "No first-order proof with the lemmas supplied"))
   end
   handle METIS (loc, msg) =>
          case fallback_type_encs of