Don't mark the user as connected until successfull connection

The idea behind distinguishing between the user being connected and
the actual connection having been made was to support retries while
the user decided to connect, even if the connection wasn't made yet.

The problem is that _user_connected is used directly to tell whether
the sender is connected or not which will be wrong if an exception
occurs on the initial connection. Since the logic retry isn't used
there we can simply mark as connected once a successfull connection
is made.

telethon/network/mtprotosender.py

@@ -136,8 +136,8 @@ class MTProtoSender:
             return
 
         self._connection = connection
-        self._user_connected = True
         await self._connect()
+        self._user_connected = True
 
     def is_connected(self):
         return self._user_connected