Add assert to check output of upgrade
Ckeck the output of ipa-server-upgrade script for error.
Related to: https://pagure.io/freeipa/issue/7644
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>