recovered error to finish proof (e.g. bad obtain export) from 223f18cfbb32;
tuned message;

--- a/src/Pure/Isar/proof.ML	Sun Dec 02 22:20:12 2012 +0100
+++ b/src/Pure/Isar/proof.ML	Mon Dec 03 14:44:00 2012 +0100
@@ -1059,7 +1059,7 @@
     val testing = Unsynchronized.ref false;
     val rule = Unsynchronized.ref (NONE: thm option);
     fun fail_msg ctxt =
-      "Local statement will fail to refine any pending goal" ::
+      "Local statement fails to refine any pending goal" ::
       (case ! rule of NONE => [] | SOME th => [Proof_Display.string_of_rule ctxt "Failed" th])
       |> cat_lines;
 
@@ -1072,7 +1072,7 @@
         writeln (Markup.markup Markup.state (Proof_Display.string_of_rule ctxt "Successful" th))
       else ();
     val test_proof =
-      try (local_skip_proof true)
+      local_skip_proof true
       |> Unsynchronized.setmp testing true
       |> Exn.interruptible_capture;
 
@@ -1085,8 +1085,7 @@
     |> local_goal print_results prep_att prepp "show" before_qed after_qed' stmt
     |> int ? (fn goal_state =>
       (case test_proof (map_context (Context_Position.set_visible false) goal_state) of
-        Exn.Res (SOME _) => goal_state
-      | Exn.Res NONE => error (fail_msg (context_of goal_state))
+        Exn.Res _ => goal_state
       | Exn.Exn exn => raise Exn.EXCEPTIONS ([exn, ERROR (fail_msg (context_of goal_state))])))
   end;