engine: Rephrase confusing audit log message

Previous audit log message contained a contradiction. It proposed that
disk was successfully removed but with a storage failure.

Change-Id: I3544d1fde9bb33422175e3212d87c0d190870ff4
Bug-Url: https://bugzilla.redhat.com/1045842
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>