Deleting a non-existent key through s3api's delete-objects should return a valid response..

```
aws s3api delete-objects --bucket my-bucket --delete '{\"Objects\":[{\"Key\":\"my-key\"}]}'
```
Gives...
```
{
    "Deleted": [
        {
            "Key": "my-key"
        }
    ]
}
```

Deleting a non-existent key through s3api's delete-objects on LocalStack returns an error response without any additional information beyond the key.

```
aws s3api delete-objects --bucket my-bucket --delete '{"Objects":[{"Key":"my-key"}]}' --endpoint-url http://localhost:4572
```
Gives...
```
{
    "ResponseMetadata": {
        "HTTPStatusCode": 200,
        "HTTPHeaders": {
            "server": "BaseHTTP/0.6 Python/3.7.5",
            "date": "Fri, 20 Dec 2019 16:02:39 GMT",
            "content-type": "application/xml; charset=utf-8",
            "content-length": "180",
            "access-control-allow-origin": "*",
            "last-modified": "Fri, 20 Dec 2019 16:02:39 GMT",
            "access-control-allow-methods": "HEAD,GET,PUT,POST,DELETE,OPTIONS,PATCH",
            "access-control-allow-headers": "authorization,content-type,content-md5,cache-control,x-amz-content-sha256,x-amz-date,x-amz-security-token,x-amz-user-agent,x-amz-target,x-amz-acl,x-amz-version-id,x-localstack-target,x-amz-tagging",
            "access-control-expose-headers": "x-amz-version-id"
        },
        "RetryAttempts": 0
    },
    "Errors": [
        {
            "Key": "my-key"
        }
    ]
}
```

Deleting an existent key seems to work fine.
bucket_name = normalize_bucket_name(bucket_name)
    
    if isinstance(to_str(encryption), six.string_types):
        encryption = xmltodict.parse(encryption)
    BUCKET_ENCRYPTIONS[bucket_name] = encryption
    return 200
def set_object_lock(bucket_name, lock_config):
    bucket_name = normalize_bucket_name(bucket_name)
    
    if isinstance(to_str(lock_config), six.string_types):
        lock_config = xmltodict.parse(lock_config)
    OBJECT_LOCK_CONFIGS[bucket_name] = lock_config
    return 200
def strip_chunk_signatures(data):
    
    
    #
    
    
    
    data_new = re.sub(b'(^|\r\n)[0-9a-fA-F]+;chunk-signature=[0-9a-f]{64}(\r\n)(\r\n$)?', b'',
        data, flags=re.MULTILINE | re.DOTALL)
    return data_new
def bucket_exists(bucket_name):