Fix exception handling for help channel pins

[MarkKoz]

Sentry Issue: BOT-7J

Discord's API returns a 400 with error code 10008 rather than returning a 404 (AKA NotFound), which is currently being caught. An unhanded error here means some stuff for tracking stats doesn't get set.

HTTPException: 400 Bad Request (error code: 10008): Unknown Message
  File "discord/client.py", line 304, in _run_event
    await coro(*args, **kwargs)
  File "bot/cogs/help_channels.py", line 701, in on_message
    await message.pin()
  File "discord/message.py", line 885, in pin
    await self._state.http.pin_message(self.channel.id, self.id, reason=reason)
  File "discord/http.py", line 225, in request
    raise HTTPException(r, data)

Unhandled exception in on_message.

[MarkKoz]

This was fixed in f97defc.