only print one error message in topic updater

[minneelyyyy]

This fixes what I perceive to be a flaw where the bot will flood the console if it doesn't have the permission to manage channels.

It replaces that with the following message

This PR also necessarily changed DiscordUtil.setTextChannelTopic to throw a PermissionException when your bot lacks permission to set the channel settings, instead of printing a generic message for that case. Which, I think is slightly better anyway, but it did require touching code in DiscordSRV as well.