feat(config): allow invalid server filters (#2419)

- Disallow setting a server filter when there is no choice available
- Allow setting an invalid server filter when there is at least one choice available
- Log at warn level when an invalid server filter is set
- Fix #2337

cmd/gluetun/main.go

@@ -249,7 +249,7 @@ func _main(ctx context.Context, buildInfo models.BuildInformation,
 		return fmt.Errorf("checking for IPv6 support: %w", err)
 	}
 
-	err = allSettings.Validate(storage, ipv6Supported)
+	err = allSettings.Validate(storage, ipv6Supported, logger)
 	if err != nil {
 		return err
 	}