Fail Rock swapout if the disk dropped some of the write requests (#352)

Fail Rock swapout if the disk dropped some of the write requests (#352)

Detecting dropped writes earlier is more than a TODO: If the last entry
write was successful, the whole entry becomes available for hits
immediately.  IpcIoFile::checkTimeouts() that runs every 7 seconds
(IpcIoFile::Timeout) would eventually notify Rock about the timeout,
allowing Rock to release the failed entry, but that notification may
be too late.

The precise outcome of hitting an entry with a missing on-disk slice is
unknown (because the bug was detected by temporary hit validation code
that turned such hits into misses), but SWAPFAIL is the best we could
hope for.