ISIS-786: event subscribers can now throw a RecoverableException or a NonRecoverableException.

... this will veto the transaction.

Any other exception is ignored.

In addition:
- fixed the todo app's demo of RecoverableExceptionAutoEscalated.
ISIS-787: change Wicket viewer OK button to using ajax

... this seems to fix the regression that has occurred in snapshot (perhaps as result of upgrade to Wicket 6.15.0?)