<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->

# Type of request: This is a ...

[ X ] bug report
[ ] feature request

# Detailed description

On BatchWriteItem if items are throttled, we are expected to receive a list of `UnprocessedItems` in the response instead of HTTP 500

## Expected behavior

Receive HTTP 200 but with Unprocessed Items in the response: https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_BatchWriteItem.html#DDB-BatchWriteItem-response-UnprocessedItems

## Actual behavior

Receive HTTP 500 ProvisionedThroughPutException. "All or nothing"

# Steps to reproduce

Start LocalStack with DynamoDb Service and "DYNAMODB_ERROR_PROBABILITY=1.0"

## Command used to start LocalStack

```
new LocalStackContainer(DockerImageName.parse("localstack/localstack:0.12.9"))
      .withEnv("DYNAMODB_ERROR_PROBABILITY", "1.0")
      .withServices(DYNAMODB)
```

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)

You can just Randomly generate a big list of objects (exceeding 1kb per batch) and check this behaviour


new_record['eventID'] = short_uid()
                new_record['eventName'] = 'MODIFY'
                new_record['dynamodb']['Keys'] = keys
                new_record['dynamodb']['OldImage'] = self._thread_local('existing_items')[i]
                new_record['dynamodb']['NewImage'] = updated_item
                new_record['eventSourceARN'] = aws_stack.dynamodb_table_arn(table_name)
                new_record['dynamodb']['SizeBytes'] = len(json.dumps(updated_item))
                records.append(new_record)
                i += 1
            delete_request = request.get('Delete')
            if delete_request:
                table_name = delete_request['TableName']
                keys = delete_request['Key']
                existing_item = self._thread_local('existing_items')[i]
                if isinstance(keys, Response):
                    return keys
                stream_spec = dynamodb_get_table_stream_specification(table_name=table_name)
                if stream_spec:
                    record['dynamodb']['StreamViewType'] = stream_spec['StreamViewType']
                new_record = clone(record)
                new_record['eventID'] = short_uid()
                new_record['eventName'] = 'REMOVE'
                new_record['dynamodb']['Keys'] = keys
                new_record['dynamodb']['OldImage'] = existing_item
                new_record['dynamodb']['SizeBytes'] = len(json.dumps(existing_item))
                new_record['eventSourceARN'] = aws_stack.dynamodb_table_arn(table_name)
                records.append(new_record)
                i += 1
        return records
    def prepare_records_to_forward_to_ddb_stream(self, records):