Add hints for when File.listFiles() returns null

Under some circumstances, File.listFiles() can return null as shown by AS7-3808:

@return  An array of abstract pathnames denoting the files and
         directories in the directory denoted by this abstract
         pathname.  The array will be empty if the directory is
         empty.  Returns <code>null</code> if this abstract pathname
         does not denote a directory, or if an I/O error occurs.

In order to get more information, if listFiles() returns null, add some checks to see if File.exists() and if File.isDirectory(). This might give us more of a clue of what happened. Also, although not likely, it might be worth to check permissions via File.canRead() and File.canWrite() too.