Elevate volume/snap "is busy" log message for volume/snap_delete

If deletion fails because the volume is busy, log at "error" level
rather than "debug" to ensure that the error is logged and visible
to a user.

Change-Id: I04031ee6b1f6647a126baa8a29c15a00b84b27b1
Merge "Elevate volume/snap "is busy" log message for volume/snap_delete"