say: on_command_error listener doesn't accept unhandled_by_cog argument

[synrg]

I discovered that when another cog (tags) dispatches a command_error, the say cog receives it and throws these lines of traceback:

Ignoring exception in on_command_error
Traceback (most recent call last):
  File "/home/synrg/.local/lib/python3.8/site-packages/discord/client.py", line 343, in _run_event
    await coro(*args, **kwargs)
TypeError: on_command_error() got an unexpected keyword argument 'unhandled_by_cog'

I talked to @phenom4n4n about this and they have an idea for a PR.