Handle empty /var/www/html

[macropin]

Does not nicely handle when /var/www/html is empty

Unsetting HOME
Traceback (most recent call last):
  File "/usr/local/bin/voltgrid.py", line 339, in <module>
    main(argv=sys.argv)
  File "/usr/local/bin/voltgrid.py", line 327, in main
    m.handle()
  File "/usr/local/bin/voltgrid.py", line 245, in handle
    shutil.rmtree(mount_dir)
  File "/usr/lib64/python2.7/shutil.py", line 256, in rmtree
    onerror(os.rmdir, path, sys.exc_info())
  File "/usr/lib64/python2.7/shutil.py", line 254, in rmtree
    os.rmdir(path)
OSError: [Errno 16] Device or resource busy: '/var/www/html'