all: Get rid of fatal logging (#5537)

* cleanup Fatal in lib/config/config.go

* cleanup Fatal in lib/config/folderconfiguration.go

* cleanup Fatal in lib/model/model.go

* cleanup Fatal in cmd/syncthing/monitor.go

* cleanup Fatal in cmd/syncthing/main.go

* cleanup Fatal in lib/api

* remove Fatal methods from logger

* lowercase in errors.Wrap

* one less channel

cmd/syncthing/gui.go

@@ -78,7 +78,8 @@ type apiService struct {
 	stop               chan struct{} // signals intentional stop
 	configChanged      chan struct{} // signals intentional listener close due to config change
 	started            chan string   // signals startup complete by sending the listener address, for testing only
-	startedOnce        chan struct{} // the service has started successfully at least once
+	startedOnce        chan struct{} // the service has started at least once
+	startupErr         error
 	cpu                rater
 
 	guiErrors logger.Recorder
@@ -174,6 +175,11 @@ func newAPIService(id protocol.DeviceID, cfg configIntf, httpsCertFile, httpsKey
 	return service
 }
 
+func (s *apiService) WaitForStart() error {
+	<-s.startedOnce
+	return s.startupErr
+}
+
 func (s *apiService) getListener(guiCfg config.GUIConfiguration) (net.Listener, error) {
 	cert, err := tls.LoadX509KeyPair(s.httpsCertFile, s.httpsKeyFile)
 	if err != nil {
@@ -236,14 +242,15 @@ func (s *apiService) Serve() {
 			// We let this be a loud user-visible warning as it may be the only
 			// indication they get that the GUI won't be available.
 			l.Warnln("Starting API/GUI:", err)
-			return
 
 		default:
 			// This is during initialization. A failure here should be fatal
 			// as there will be no way for the user to communicate with us
 			// otherwise anyway.
-			l.Fatalln("Starting API/GUI:", err)
+			s.startupErr = err
+			close(s.startedOnce)
 		}
+		return
 	}
 
 	if listener == nil {
@@ -410,6 +417,19 @@ func (s *apiService) Serve() {
 	}
 }
 
+// Complete implements suture.IsCompletable, which signifies to the supervisor
+// whether to stop restarting the service.
+func (s *apiService) Complete() bool {
+	select {
+	case <-s.startedOnce:
+		return s.startupErr != nil
+	case <-s.stop:
+		return true
+	default:
+	}
+	return false
+}
+
 func (s *apiService) Stop() {
 	close(s.stop)
 }