aci/layout: add old ACVersion error type
aci/layout: add old ACVersion error type

This allows callers of Validate functions to handle this kind of
error specially.

dustymabe/spec,krnowak/appc-spec,craimbert/spec,squeed/spec,apcera/appc-spec,ChengTiesheng/spec,endocode/appc-spec,jonboulle/spec,zenlinTechnofreak/spec,endocode/appc-spec,alban/appc-spec,squeed/spec,zenlinTechnofreak/spec,craimbert/spec,craimbert/spec,dgonyeo/spec,appc/spec,iaguis/appc-spec,marineam/spec,jonboulle/spec,dgonyeo/spec,jonboulle/spec,zenlinTechnofreak/spec,appc/spec,apcera/appc-spec,squeed/spec,stufflebear/spec,ChengTiesheng/spec,alban/appc-spec,marineam/spec,ChengTiesheng/spec,dustymabe/spec,stufflebear/spec,dgonyeo/spec,3ofcoins/appc-spec,apcera/appc-spec,alban/appc-spec,marineam/spec,stufflebear/spec,3ofcoins/appc-spec,endocode/appc-spec,krnowak/appc-spec,iaguis/appc-spec,3ofcoins/appc-spec,appc/spec,dustymabe/spec,krnowak/appc-spec,iaguis/appc-spec