sync logic can lose images

[donpdonp]

right now the sync logic will happily mark a photo as backed up when the filesystem save fails. if someone were to run sync to completion and delete their flickr account, they would find no images!

I ran into this when config.yml didnt have the trailing slash for the image folder. 1) validate the destination folder right at the start of the sync process. 2) abort sync on file save fail.