do not complain about missing objectives files

Manually added objectives are getting rarer now. Since the auto generation of objectives is getting better and beter.

Util/Config.cs

@@ -81,6 +81,9 @@ namespace EliteBGS.Util {
         }
 
         public void LoadObjectives(Report report) {
+            if (!File.Exists(objectives_file)) {
+                return;
+            }
             var serializer = JsonSerializer.CreateDefault();
             using (var file = new StreamReader(File.OpenRead(objectives_file), Encoding.UTF8)) {
                 var stream = new JsonTextReader(file);