text: Title: Add a BUSYGROUP reply error
  Body: The XGROUP CREATE command can return a BUSYGROUP error when a group already exists: https://redis.io/commands/xgroup
  
  I think the `ReplyError` subclass for matching it would look like this:
  
  ```py
  class BusyGroupError(ReplyError):
      MATCH_REPLY = "BUSYGROUP Consumer Group name already exists"
  ```