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/helpers.py

@@ -72,6 +72,21 @@ def strip_text(text, entities):
     return text
 
 
+def retry_range(retries):
+    """
+    Generates an integer sequence starting from 1. If `retries` is
+    negative or ``None`` then sequence is infinite, otherwise it will
+    end at `retries + 1`.
+    """
+    yield 1
+    if retries is None:
+        retries = -1
+    attempt = 0
+    while attempt != retries:
+        attempt += 1
+        yield 1 + attempt
+
+
 # endregion
 
 # region Cryptographic related utils