Warnings sheet

[braxtonhall]

Problem: if we detect a problem anywhere in the tag index (like duplicate tags, broken formatting in the TagIndexIndex) we need a way to report it to one of the co-librarians to fix

Solution: Create a hidden sheet dedicated to reporting warnings as they are detected

A specific problem should be reported at most once, so a warning needs to check if it's already present before appending to the warning page.

The warning sheet should also have a dedicated spot for a librarian to put their contact email, so the bot can @ the librarian and they will get an email when a new problem arises

[braxtonhall]

Great for the sheet being out of date
#261