title: Discord.py ctx.message.delete() raises error in cog
body: <p>I recently moved my bot into cogs, but with that came some troubles. I have a command that will make the bot repeat a message when you invoke the command. I want the bot to delete the command that invokes the message and then send the message you typed. I have this (note: this worked when it wasn't in a cog):</p>
  
  <pre><code>@commands.command()
      async def say(self, ctx, *, arg=None):
          if arg is None:
              await ctx.send(f'{ctx.message.author.mention} Tell me what to say!')
          if len(arg) &gt; 50:
              await ctx.send(f"{ctx.message.author.mention} That's too long!")
          else:
              await ctx.message.delete()
              await ctx.send(f'''{arg}
      - {ctx.message.author.mention}''')
  </code></pre>
  
  <p>The code runs and deletes the message that invoked the command and sends the message, but it still throws an error:</p>
  
  <pre><code>discord.errors.NotFound: 404 NOT FOUND (error code: 10008): Unknown Message
  </code></pre>
  
  <p>Any ways to fix this? Again the command works but it still throws and error.</p>
  
  <p>Thanks in advance.</p>
  
  <p>EDIT: After some testing I think I have found the issue. Another problem with my code is resulting in this issue, this code in itself isn't causing the error.</p>
  
  <p>Just so if others are having a similar issue when moving to cogs, my code had an on_message listener in a cog. I mistakenly added a <code>await self.client.process_commands(message)</code></p>
  
  <p>You do not need this in a cog.</p>