Align logback's behavior with log4j2 when getting a logger that doesn't exist

Currently, if a `GET` request to the `/loggers` endpoint is made for a logger that doesn't exist, logback will create the logger. Log4j2 returns a 404. It would be nice to align the two. Logback has a `exists()` method which we can check before returning the logger configuration. This would also prevent a `GET` request from having a side-effect.