Quiet down swap out error reporting.

Quiet down swap out error reporting.

Do not report swap out errors at level 1. When things go wrong, the already
bad situation is made worse by writing lots of error messages to cache.log.

Do not report system error because the errno may be stale or irrelevant.
If error details are needed, the code should save and propagate the actual
errno in addition to the DISK_ERROR or similar status.