Feature/graviton event

What do you think about:
- making the RestException abstract, since I think it should not be used directly
- changing the consructor of every extending exception to
```
public function __construct($message = "Deserialization Error", $prev = null) 
{
    parent::__construct($message, 500, $prev);
}
```
- declaring the extending exceptions as »final« to encourage the use of the open-close-principle.