#532 ugly handling if download folder is removed

[Donovan Eastman]

If the designated download folder has been removed or
is otherwise unavailable Juice appears to do nothing
when you try to download files. The only evidence that
anything has happened is the following log entry:

Uncaught exception!
Traceback (most recent call last):
File "ipodder\core.pyc", line 941, in start
File "ipodder\core.pyc", line 597, in download_enclosures
File "ipodder\misc.pyc", line 23, in freespace
OSError: [Errno 13] Permission denied: u'D:\\podcasts'

It would be nice to get a more meaningful error message
here.