Tumblr improvements

While importing my tumblr, I ran into a number of reblogged posts that had bad photos in them.  This PR includes fixes to retry smaller photos if one fails.  I also made the importer a bit more verbose so I could keep track of it working.  For folks with lots of photos in their blogs, I made sure that those photos already saved locally are not re-downloaded, making the importer run a lot faster on re-runs.