Introduce exceptions CoverArtImageIOError and CoverArtImageIdentificationError

- hide imageinfo exceptions
- hide io/filesystem exceptions
- those may change in future, callers don't need to know details
Re-add dummy load to ensure test images are > 64kb

As said in comment, size was increased to catch a bug in mutagen, so better
to keep it.
Improve debugging messages
Fix up set_data()
Output more infos about downloaded image in debug mode
PEP8 fixes
Move test for save_images_to_tags option to metadata.images_to_be_saved_to_tags
Drop index/enumerate(), unused now