<!-- 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


""" Returns True if host responds to a ping request """
    is_windows = platform.system().lower() == 'windows'
    ping_opts = '-n 1' if is_windows else '-c 1'
    args = 'ping %s %s' % (ping_opts, host)
    return subprocess.call(args, shell=not is_windows, stdout=subprocess.PIPE, stderr=subprocess.PIPE) == 0
def in_docker():