Failure to validate `subdirs` when `indludes_subdirs = true` in `lotman_update_lot_usage_by_dir`

[jhiemstrawisc]

When one of the input objects in a directory update object has the form:

{
    "includes_subdirs": true,
    "path": "lot2",
    "size_GB": 1234,
}

a cryptic error is thrown: [json.exception.type_error.305] cannot use operator[] with a string argument with number

This happens because the includes_subdirs = true causes LotMan to look for the subdirs key, which isn't present in this case.

We should handle this error and do a better job validating the input.