lib/syncthing: Prevent hangup on error during startup (fixes #6043) (#6047)

cmd/syncthing/main.go

@@ -647,7 +647,9 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 		}
 	}
 
-	app.Start()
+	if err := app.Start(); err != nil {
+		os.Exit(int(syncthing.ExitError))
+	}
 
 	cleanConfigDirectory()