How to use the metalsmith.exceptions.Error function in metalsmith

_validate_instances(self.instances)
        except Exception as exc:
            LOG.error('Failed to validate provided instances. %s', exc)
            return actions.Result(error=six.text_type(exc))

        provisioner = _provisioner(context)

        not_found = []
        found = []
        for request in self.instances:
            ident = request.get('name', request['hostname'])

            try:
                instance = provisioner.show_instance(ident)
            # TODO(dtantsur): replace Error with a specific exception
            except (sdk_exc.ResourceNotFound, metalsmith.exceptions.Error):
                not_found.append(request)
            except Exception as exc:
                message = ('Failed to request instance information for %s'
                           % ident)
                LOG.exception(message)
                return actions.Result(
                    error="%s. %s: %s" % (message, type(exc).__name__, exc)
                )
            else:
                # NOTE(dtantsur): metalsmith can match instances by node names,
                # provide a safeguard to avoid conflicts.
                if (instance.hostname and
                        instance.hostname != request['hostname']):
                    error = ("Requested hostname %s was not found, but the "
                             "deployed node %s has a matching name. Refusing "
                             "to proceed to avoid confusing results. Please "