Suppress NotFound for batch deletion.

[jchristgit]

Fixes BOT-1MF

I will copy my comment from the code, as it explains why we want this:

In the rare case where we found messages matching the spam filter across
multiple channels, it is possible that a single channel will only
contain a single message to delete. If that should be the case,
discord.py will use the "delete single message" endpoint instead of the
bulk delete endpoint, and the single message deletion endpoint will
complain if you give it that does not exist. As this means that we have
no other message to delete in this channel (and message deletes work
per-channel), we can just log an exception and carry on with business.

[jchristgit]

Traceback found via pod logs:

2021-09-24 16:34:34 | bot | ERROR | Unhandled exception in on_message.
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/discord/client.py", line 343, in _run_event
    await coro(*args, **kwargs)
  File "/bot/bot/exts/filters/antispam.py", line 220, in on_message
    await self.maybe_delete_messages(relevant_messages)
  File "/bot/bot/exts/filters/antispam.py", line 253, in maybe_delete_messages
    await channel.delete_messages(messages)
  File "/usr/local/lib/python3.9/site-packages/discord/channel.py", line 294, in delete_messages
    await self._state.http.delete_message(self.id, message_id)
  File "/usr/local/lib/python3.9/site-packages/discord/http.py", line 250, in request
    raise NotFound(r, data)
discord.errors.NotFound: 404 Not Found (error code: 10008): Unknown Message

[jchristgit]

(This probably also fixes some Sentry issue, but I don't have access to anything there, I will request access to the devops team there)

[ChrisLovering]

(This probably also fixes some Sentry issue, but I don't have access to anything there, I will request access to the devops team there)

added sentry ref to description