lib/config: Remove "Invalid" attribute (fixes #2471)

This contains the following behavioral changes:

 - Duplicate folder IDs is now fatal during startup
 - Invalid folder flags in the ClusterConfig is fatal for the connection
   (this will go away soon with the proto changes, as we won't have any
   unknown flags any more then)
 - Empty path is a folder error reported at runtime

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3370

cmd/syncthing/main.go

@@ -863,7 +863,6 @@ func loadConfig() (*config.Wrapper, error) {
 	cfg, err := config.Load(cfgFile, myID)
 
 	if err != nil {
-		l.Infoln("Error loading config file; using defaults for now")
 		myName, _ := os.Hostname()
 		newCfg := defaultConfig(myName)
 		cfg = config.Wrap(cfgFile, newCfg)