ISIS-351: services in applib, redirect to error page

* added ExceptionRecognizer interface in applib, along with various helper implementations
* enhanced ServicesInjector (in core) to support multiple services of given type
* enhanced Wicket's ErrorPage to also render simple messages (ie suppress stacktrace for recognized exceptions)
* hooked WebRequestCycleForIsis to look for ExceptionRecognizers and redirect to ErrorPage accordingly
* provided implementations within the JDO object store
* updated ToDo app to demonstrate
* fixed rendering of select2 drop-down choices in Wicket viewer