change throw mechanism to fit better with cleanup mechanism:

- balls are now stored off line and recovered by Prolog code when everything
is safe.
- if a ball exists, throw uses the *existing* one (be careful not to have one
laying around).
- Jump routine cleans up every cp except ones for setup_call handling, backtracking simply calls setup handlers.

C/c_interface.c

@@ -1940,6 +1940,7 @@ YAP_GoalHasException(Term *t)
 X_API void
 YAP_ClearExceptions(void)
 {
+  Yap_DeleteGlobal(AtomCatch);
   EX = 0L;
   UncaughtThrow = FALSE;
 }