ExtensionsExtension throws insufficient exception

Version: 3.0.1

I have following code
```php
use Nette\DI\CompilerExtension;

final class NotFoundExtension extends CompilerExtension {
```

ExtensionsExtension throws `Extension should be Nette\DI\CompilerExtension, 'NotFoundExtension' given.`

Expected something like: `Extension 'NotFoundExtension' was not found`
