turn returned errors.New into variables

auth/auth.go

cmd/rednerd/cmd/admin_password.go

engines/mjml/engine.go

engines/wpd/wpd.go

lib/redner.go

rendering/engine_registry.go

rendering/pooled_engine.go

restapi/handlers/account-template-add.go

restapi/handlers/account-template-update.go

restapi/handlers/auth-userinfo.go

restapi/handlers/render.go

restapi/handlers/template-varlist.go

restapi/server_.go

utils/log-console-writer.go

utils/parse_period.go

utils/parse_period_test.go