"skipped" sectors must be replaced with a substitute "good" sector, or the
entire partition must be skipped. They should not just be omitted.
This patch also fixes the test to verify the _entire_ proof instead of just
verifying that the proof includes the correct sectors.
We were ignoring quite a few error cases, and had one case where we weren't
actually updating state where we wanted to. Unfortunately, if the linter doesn't
pass, nobody has any reason to actually check lint failures in CI.
There are three remaining XXXs marked in the code for lint.
