Indexing a folder with a subfolder that a user does not have permission to access #358

Description

One of the DASCore manuscript reviewers noted the following:

For some specific cases, DASCore is not able to index the data folder. For example, a permission denied error is raised if the folder contains another subdirectory which the user does not has permission on.

I think this is a bit of an unusual scenario we simply haven't encountered before. At first glance, it will probably just be a matter of including IOError in this try except block, but we need to figure out how to include an automated test for this behavior.