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# 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


global SERVICE_PORTS, CONFIG_ENV_VARS
    SERVICE_PORTS = service_ports or parse_service_ports()
    globs = globals()
    protocol = get_protocol()
    
    for key, value in six.iteritems(DEFAULT_SERVICE_PORTS):
        key_upper = key.upper().replace('-', '_')
        
        port_var_name = 'PORT_%s' % key_upper
        port_number = service_port(key)
        globs[port_var_name] = port_number
        url = '%s://%s:%s' % (protocol, LOCALSTACK_HOSTNAME, port_number)
        
        url_key = 'TEST_%s_URL' % key_upper
        
        existing = os.environ.get(url_key)
        url = existing or url
        
        globs[url_key] = url
        
        os.environ[url_key] = url
    
    os.environ['LOCALSTACK_HOSTNAME'] = LOCALSTACK_HOSTNAME
    
    CONFIG_ENV_VARS += ['LOCALSTACK_' + v for v in CONFIG_ENV_VARS if not v.startswith('LOCALSTACK_')]
    CONFIG_ENV_VARS = list(set(CONFIG_ENV_VARS))
def service_port(service_key):