libct/cg/sd: add isDbusError
libct/cg/sd: add isDbusError

Generalize isUnitExists as isDbusError, and use errors.As while at it
(which can handle wrapped errors as well).

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

kolyshkin/runc,kolyshkin/runc,opencontainers/runc,kolyshkin/runc,opencontainers/runc,dqminh/runc,rhatdan/runc,rhatdan/runc,dqminh/runc,rhatdan/runc,opencontainers/runc,kolyshkin/runc,opencontainers/runc,dqminh/runc,rhatdan/runc,dqminh/runc