fix: panic on END_BLOCK error

Right now if we fail to rewind the SwingSet state, we return an
error.  The best reaction is to panic so that we can replay the
state from scratch, and get to the new state.

packages/cosmic-swingset/x/swingset/blockers.go

@@ -66,7 +66,9 @@ func EndBlock(ctx sdk.Context, req abci.RequestEndBlock, keeper Keeper) ([]abci.
 
 	// fmt.Fprintln(os.Stderr, "Returned from SwingSet", out, err)
 	if err != nil {
-		return nil, err
+		// NOTE: A failed END_BLOCK means that the SwingSet state is inconsistent.
+		// Panic here, in the hopes that a replay from scratch will fix the problem.
+		panic(err)
 	}
 	return []abci.ValidatorUpdate{}, nil
 }