Check category_tree datasets synchronisation
Two more checks in validate-storage.py:
- checks if a dataset declared in category_tree.json is present on disk
- error_code: 
dataset-not-found-on-storage 
 - error_code: 
 - checks if a dataset present in json-data folder is referenced in category_tree.json
- error_code: 
dataset-not-found-in-category-tree 
 - error_code: 
 
This MR was inspired by this issue