preflight refuses a layout v0.16 cannot represent, but it reports the problem
rather than the shape of the instance around it. Choosing between giving a
tenant its own domains, moving accounts, or collapsing everything into one
tenant needs the whole picture: which domains have an owner, which accounts
reach across to somebody else's, and which of those the converter would have
repaired by itself.
Read-only, and it names the offending domain against each account that
touches it rather than making the reader join the two lists by eye.
