Check if folders exist before trying to delete them

[ledigchr]

Check that output folders exist before trying to delete them. Only relevant when --cleanup is used. Does not influence results, but can cause an error during the final cleanup.

[schuhschuh]

Alternatively, no need to query file system twice, just catch exception:

def try_rmdir(path):
    try:
        os.rmdir(path)
    except IOError:
        pass