Better messaging around image upload cleanup

Image upload requires making a disk to bulk write to, and then a snapshot, and then that's made into an image. At the end, we have the image, and we can delete the disk and snapshot.

1. We don't explain what these cleanup steps are. It would be nice to either have a tooltip on those steps explaining why or at least link to the image upload guide, which explains it.
2. If cleanup fails but the image got created, the user has mostly done what they came to do, so instead of getting stuck on the cleanup steps, we could say

   Done!*
   
   * Your image is uploaded but the disk and snapshot we use to make it are still hanging around. You might want to go delete them.
   

Obviously the underlying issue causing cleanup to fail is more important, but we can handle it nicely here. Thanks @jordanhendricks for the suggestion.

Start by locating the image upload flow in the console and compare its cleanup steps with the linked image upload guide. The work is done when those steps explain or link to the cleanup rationale and a successful image upload reports completion clearly even if disk or snapshot cleanup fails.