Better error message when extension is not properly configured

Happens since version 2.1.

If you don't properly configure database connection (or have same sample configuration as are defaults in extension), DI compilation produces very weird and missleading error 

```
Nette\DI\ServiceCreationException

Service '...': Reference to missing service of type Kdyby\Doctrine\EntityManager.
```

this happens because EM is not registred by extension, because `$config` in [here](https://github.com/Kdyby/Doctrine/blob/master/src/Kdyby/Doctrine/DI/OrmExtension.php#L143) is empty.


It was introduced by this commit https://github.com/Kdyby/Doctrine/commit/c9450ab6a8c94f112a57f93d20df35f695669b06