Explicitly handle negative space savings

[Ravna]

printSavedSpace or formatByteCount should probably check for negative numbers:

numfmt: invalid option -- '4'
Try 'numfmt --help' for more information.
=> Docker images: Saved

It's not clear what ate some space while deleting zero docker images, but the result caused some raised eyebrows.
(Perhaps something else during our build was racing, or perhaps docker image prune --all --force creates some tempfiles.
I'll note that this doesn't seem to happen every time.)