Implementing a way to get some useful debug info about why a form was bl...

...ackholed
@lorenzo removed it. @AD7six I've added that info instead.

```
Blackhole Error
Error: The request has been black-holed	
Blackhole Scope: auth	
Dirty Fields
EventRegistration.agenda_item_id => 423

Notice: If you want to customize this error message, create app\View\Errors\fatal_error.ctp
```