[dx] Include skipped class name in non-Rector-rule warning message

[TomasVotruba]

The warning for ->withSkip() classes that are not Rector rules printed the class names in a separate listing block below the message. The message itself had no reference to the broken value.

Now the class name is part of the message:

-This skipped class is not a Rector rule, so it can never be skipped. Only classes that
-implement "Rector\Contract\Rector\RectorInterface" can be used in "->withSkip()"
-
- * App\Service\SomeService
+This skipped class "App\Service\SomeService" is not a Rector rule, so it can never be
+skipped. Only classes that implement "Rector\Contract\Rector\RectorInterface" can be
+used in "->withSkip()"

The now redundant listing() call is removed.