tumblr_backup: URLErrors are also IOErrors
tumblr_backup: fix thread termination issues

There's no way around an event that can be set to tell the threads to
stop. It may take a while, though, because each thread will finish
saving its current post with all images before quitting.

That's why there's this cute cleanup animation.
tumblr_backup: document option -k
tumblr_backup: explain about duplicate inline image names