[MailSystem] Proper try/excepts cache func

[SharkyTheKing]

I realized after a quick surf that I failed to have proper try/except checks in the cache function (_update_cache), which it'll error if the channel is not seen.

If someone would want to work on this, please feel free to.

Few things to look at:

• Attempt to get channel first, if failed try/except on fetching the channel.
• If failed on fetch, remove from config.