Add notification to io-spare.sh

The io-spare.sh ZEDLET does not generate a notification when a failing
device is replaced with a hot spare. Maybe it should tell someone.

This commit adds a notification message to the io-spare.sh ZEDLET.
This notification is triggered when a failing device is successfully
replaced with a hot spare after encountering a checksum or io error.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>