core: Audit log failed external scheduler discovery

When the external scheduler thread fails discovery no further
external operations occure.
Now we let the user know about it.

Change-Id: I198b4fd6300d903fcce55afc400a3075af1ea11c
Bug-Url: https://bugzilla.redhat.com/1001986
Signed-off-by: Noam Slomianko <nslomian@redhat.com>