PerformInCategory performs a PageFunction
(function taking in a filename and returning an error)
in all pages and subcats  in a category
and returns a slice of all errors it returns.
Category need not start with "Category:".

Added more notes with OpenAndUpload & BatchUpload:
OpenAndUpload need not start with "File:"
and BatchUpload's paramFunc may have filename
set to empty string or just return nil.