Previously, we were simply discarding rows from the repair queue when
they couldn't be repaired (either because the overlay said too many
nodes were down, or because we failed to download enough pieces).
Now, such segments will be put into the irreparableDB for further
and (hopefully) more focused attention.
This change also better differentiates some error cases from Repair()
for monitoring purposes.
Change-Id: I82a52a6da50c948ddd651048e2a39cb4b1e6df5c
79553059cb