dcerpc: refactor assess_dcerpc_exception
assess_dcerpc_exception was used in multiple places with a pre-step
which was rather common. Move this to one spot.
This also fixes pylint warning about unbalanced unpacking.
https://pagure.io/freeipa/issue/6874
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>