[-] BO : Fix some bugs in the Category importer and add comments

This fixes two bugs :
- having categories ID 1 & 2 (resp. Root and Home) would always fail regardless of the "forceIDs" setting.
- imported categories would always override already existing categories.
[*] BO : Fix some localized error messages
[-] BO : Fix placeholder categories not getting overwritten when importing

This fixes a bug where the missing parent of an imported category wouldn't be rewritten when the actual import data was subsequently found, resulting in duplicate categories.