Humane disk/image sizes in error messages

Moved from https://github.com/oxidecomputer/console/issues/1049.

Disk sizes are in bytes and that makes them hard to read and work with. We could do something about this on the client, but it would be very brittle.

There's an interesting wrinkle here, which is that because what matters are byte sizes, how you should round the sizes may depend on the particular values in question. For example, if you're looking at Error: Disk size was 999 bytes, but it needs to be at least 1000 bytes, both of those would be 1 KB in a naive approach, making the message useless. So in order to do this well, the error serializer needs to take into account what is being said. (This may be an argument against doing anything about this for the time being.)

No files, tests, or entry points are named. Start by locating the error serializer and reviewing how disk and image byte sizes are rendered; done should mean error messages are more readable without collapsing distinct values such as 999 and 1000 bytes.