Improve permission-related errors (#1231)

telethon/client/chats.py

@@ -847,6 +847,9 @@ class ChatMethods:
         """
         entity = await self.get_input_entity(entity)
         user = await self.get_input_entity(user)
+        if isinstance(user, types.InputPeerSelf):
+            raise ValueError('You cannot admin yourself')
+
         if not isinstance(user, types.InputPeerUser):
             raise ValueError('You must pass a user entity')
 
@@ -996,6 +999,9 @@ class ChatMethods:
             ))
 
         user = await self.get_input_entity(user)
+        if isinstance(user, types.InputPeerSelf):
+            raise ValueError('You cannot restrict yourself')
+
         if not isinstance(user, types.InputPeerUser):
             raise ValueError('You must pass a user entity')