Having attributes in mappings that don't exist doesn't raise any error or warning.

[KhaledTo]

When specifying a model, if I add some attributes that don't exist in my dataset I get no warnings or errors. To reproduce, you can use the sample model.json file in examples/hello_world folder:

            "mappings": {
                        "item.line_item": "line_item",
                        "item.subcategory": "subcategory",
                        "item.subcategory_label": "subcategory_label",
                        "item.category": "category",
                        "item.cpu_item": "cpu_item",
                        "item.category_label": "category_label",
                        "item.year_item": "year_item",
                        "item.doesntexist1": "itemdoesntexist1",
                        "item.doesntexist2": "itemdoesntexist2"
                         },