Error handling when import directory contains only directories

Issue Summary

If the file import path contains only directories, admin/content/import-files generates a completely blank page. It is not clear from the import path setting page that there must be files in the import directory, and not just directories containing files (which is more flexible).

I would like to update the code so that when the module scans for files, it will look into directories if it does not find any files at the top level, but I'm not sure how. It seems that the module should have more graceful error handling than returning a blank page in this situation (it's alarming!) or at the very minimum, the help text for setting the import path should specify that there must be files within that directory. The current functionality is OK, but the lack of warning and error handling are a bit problematic.