Convert set name to unicode, as flickr API returns

This broke when I had a directory of photos called 25-Jañuary-2014 -
it failed to notice the set already existed and gave a warning
"Unicode equal comparison failed to convert both arguments to Unicode"

Test case:
1. add a new folder of photos with unicode in the folder name
2. upload at least one photo from this folder
3. cancel the uploads and check a new set is created with unicode name
4. start an upload session with more photos in the unicode folder
5. make sure new photos go to the existing set when upload ends

Hopefully fixes #10.
Better debug error when set edit fails
Fix for ctrl-c when the current set does not exist

This patch fixes the case when a bunch of photos would go to a new set,
but that set doesn't exist, and you ctrl-c cancel the upload. Previously
it would add the photos to some random existing set.