Check if log.txt exists before trying to open

This fixes #3

bot/__init__.py

@@ -5,8 +5,10 @@ import threading
 from telegram.ext import Updater
 import os
 
-with open('log.txt', 'r+') as f:
-    f.truncate(0)
+
+if os.path.exists('log.txt'):
+    with open('log.txt', 'r+') as f:
+        f.truncate(0)
 
 logging.basicConfig(format='%(asctime)s - %(name)s - %(levelname)s - %(message)s',
                     handlers=[logging.FileHandler('log.txt'), logging.StreamHandler()],