core: clear log message on failure during disk removal.

1. changed message.
2. reduced audit log severity.

Change-Id: I591c9f4cbb0aeb5fd6c646211e815bdd88fcadd8
Bug-Url: https://bugzilla.redhat.com/867849
Signed-off-by: Liron Aravot <laravot@redhat.com>