core: Adds Kerberos and LDAP errors stacktrace to engine log

Adds Kerberos error message and stacktrace to engine log when user
authentication to KDC failed.
Adds LDAP error stacktrace to engine log for LDAP query errors.

Change-Id: I6ccca21a70f6e7a683f49884c3c15e45b016ef40
Bug-Url: https://bugzilla.redhat.com/975777
Signed-off-by: Martin Perina <mperina@redhat.com>