Correct the error message

modules/contest.js

@@ -206,7 +206,7 @@ app.get('/contest/:id/:pid', async (req, res) => {
   try {
     let contest_id = parseInt(req.params.id);
     let contest = await Contest.fromID(contest_id);
-    if (!await contest.isRunning()) throw 'The contest has ended.'
+    if (!await contest.isRunning()) throw 'The contest has ended or not started.'
 
     let problems_id = await contest.getProblems();