A DeadPropsHolder could be backed by disk, whether directly or
indirectly via a database, so we should allow for I/O to fail.

Change-Id: Id40bcc86eb854212ef254ea1e1c54fd2a2b1960a
Reviewed-on: https://go-review.googlesource.com/10472Reviewed-by: Robert Stepanek <robert.stepanek@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>