Fetches a specific batch from the validator, specified by id.

        Request:
            path:
                - batch_id: The 128-character id of the batch to be fetched

        Response:
            data: A JSON object with the data from the fully expanded Batch
            link: The link to this exact query

async def fetch_batch(self, request):
        """Fetches a specific batch from the validator, specified by id.

        Request:
            path:
                - batch_id: The 128-character id of the batch to be fetched

        Response:
            data: A JSON object with the data from the fully expanded Batch
            link: The link to this exact query
        """
        error_traps = [error_handlers.BatchNotFoundTrap]

        batch_id = request.match_info.get('batch_id', '')
        self._validate_id(batch_id)

        response = await self._query_validator(
            Message.CLIENT_BATCH_GET_REQUEST,
            client_batch_pb2.ClientBatchGetResponse,
            client_batch_pb2.ClientBatchGetRequest(batch_id=batch_id),
            error_traps)

        return self._wrap_response(
            request,
            data=self._expand_batch(response['batch']),
            metadata=self._get_metadata(request, response))