mfs: Minor PEP-8 whitespace tweaks

Signed-off-by: David Aguilar <davvid@gmail.com>
mockfs.storage: Do not attempt to close a file that was never opened

backend.SaveFile() may raise IOError.  Catch it and set the file()
instance's closed attribute so that we do not try and close a file that
was never opened.

Signed-off-by: David Aguilar <davvid@gmail.com>