[DX] Improve error message about invalid config - indent path

[TomasVotruba]

On dev-master, I get :

Fatal error: Uncaught TypeError: Argument 1 passed to Rector\DependencyInjection\Loader\RectorServiceParametersShifter::resolveRectorConfiguration() must be of the type array, string given, called in vendor/rector/rector/src/DependencyInjection/Loader/RectorServiceParametersShifter.php on line 68 and defined in vendor/rector/rector/src/DependencyInjection/Loader/RectorServiceParametersShifter.php:109

Originally posted by @gnutix in #2041 (comment)