DRY and readability on self._error.
DRY and readability on self._error.

MacHu-GWU/cerberus-sanhe,MacHu-GWU/cerberus-sanhe,funkyfuture/cerberus,calve/cerberus,brettatoms/cerberus,funkyfuture/cerberus,pyeve/cerberus,pyeve/cerberus,nicolaiarocci/cerberus,dkellner/cerberus,nicolaiarocci/cerberus,dkellner/cerberus,calve/cerberus