Print correct subject on CA cert verification failure
In load_external_cert(), if verification fails for a certificate in
the trust chain, the error message contains the last subject name
from a previous iteration of the trust chain, instead of the subject
name of the current certificate.
To report the correct subject, look it up using the current
nickname.
Part of: https://pagure.io/freeipa/issue/7761
Reviewed-By: Christian Heimes <cheimes@redhat.com>