repart: when we can't fit the partitions in, report needed disk size current disk...

repart: when we can't fit the partitions in, report needed disk size current disk size

This improves error output in repart if we can't fit the defined
partitions into the disk image. With this change we'll now show not only
the disk size we need (as before), but also the current one, as well as
the largest free area on disk.

This should make it a bit easier to debug disk space issues that repart
runs into.