Support exclusive conversations by default

telethon/errors/common.py

@@ -79,6 +79,17 @@ class CdnFileTamperedError(SecurityError):
         )
 
 
+class AlreadyInConversationError(Exception):
+    """
+    Occurs when another exclusive conversation is opened in the same chat.
+    """
+    def __init__(self):
+        super().__init__(
+            'Cannot open exclusive conversation in a '
+            'chat that already has one open conversation'
+        )
+
+
 class MultiError(Exception):
     """Exception container for multiple `TLRequest`'s."""