Using category channel in `/add` argument results in error

[ariscript]

Description of bug
Using a category as the optional channel argument for the /add command causes an error with the message "Channel for keyword to add not cached".

To Reproduce

1. Use command /add <keyword> <link to category>

Expected behavior
I would expect either an explicit error message saying that category channels aren't allowed for the argument, or (even better) listen for the keyword in all text channels under the category.

Screenshots

[ThatsNoMoon]

This error message should be fixed as of 1a525b7. I'll leave this open as an enhancement to make it support using categories, as it will now simply ask for a text channel.