an image is loaded on a node. We want to know both success and failure over
time so that we can determine when a image works or does not work on a
particular node/type. This is primarily for the image tracker to determine
what images work on what node types, but might be useful for in other
situations. I realize this duplicates some info we already have in the
image_history table, but that does not record failure, only success, and it
mostly concerned with who is using what images.