Fix a couple of inconsistencies in the public interface (#1102)

* Create `_NOT_A_REQUEST` when needed. Currently, modifications    
  in the raised exception would be "global".    
* `retries` parameters were actually attempts. This has been fixed    
  to actually be the amount of retries, so 0 now means don't retry.    
* Helper function to deal with retries instead of using a range with    
  different styles every time.

telethon/client/account.py

@@ -53,7 +53,7 @@ class _TakeoutClient:
         wrapped = []
         for r in requests:
             if not isinstance(r, TLRequest):
-                raise _NOT_A_REQUEST
+                raise _NOT_A_REQUEST()
             await r.resolve(self, utils)
             wrapped.append(functions.InvokeWithTakeoutRequest(takeout_id, r))