[EventDispatcher] Throw exception is subscriber is miss configured

| Q             | A
| ------------- | ---
| Bug fix?      | maybe
| New feature?  | no
| BC breaks?    | maybe
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -


Lot of new comers (during trainings) made the same typo:

```php
class TestEventSubscriberTypo implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
    {
        return array('pre.foo', 'preFoo');
    }
}
```

instead of

class TestEventSubscriberTypo implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
    {
        return array('pre.foo' => 'preFoo');
    }
}
```

So I added a check.

This can be a BC break as the event name can be an int. But I think (I hope?)
nobody do that.