config: make config file relative to the config dir

a configuration parsing error is now fatal

cmd/portable.go

@@ -124,6 +124,7 @@ Please take a look at the usage below to customize the serving parameters`,
 			}
 			service := service.Service{
 				ConfigDir:     filepath.Clean(defaultConfigDir),
+				ConfigFile:    defaultConfigFile,
 				LogFilePath:   portableLogFile,
 				LogMaxSize:    defaultLogMaxSize,
 				LogMaxBackups: defaultLogMaxBackup,