Missing else in dataset.py

[hannahbnelson]

Inside dataset.py, in flatten(), there is the following block:

        if fs.isdir(entry):
            res.extend(fs.ls(entry))
        elif fs.isfile(entry):
            res.append(entry)

This is missing an else statement so if fs.isdir() and fs.isfile() are both false, there is no clear error message and this function doesn't behave correctly so it's very difficult to debug.