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.
""" Class to handle putting with urllib """
    def __init__(self, *args, **kwargs):
        return Request.__init__(self, *args, **kwargs)
    def get_method(self, *args, **kwargs):
        return 'PUT'
class S3ListenerTest(unittest.TestCase):
    def setUp(self):
        self.s3_client = aws_stack.connect_to_service('s3')
        self.sqs_client = aws_stack.connect_to_service('sqs')
    def test_create_bucket_via_host_name(self):
        body = """<?xml version="1.0" encoding="UTF-8"?>
            <CreateBucketConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
                <LocationConstraint>eu-central-1</LocationConstraint>
            </CreateBucketConfiguration>"""
        headers = aws_stack.mock_aws_request_headers('s3')
        bucket_name = 'test-%s' % short_uid()
        headers['Host'] = '%s.s3.amazonaws.com' % bucket_name
        response = requests.put(config.TEST_S3_URL, data=body, headers=headers, verify=False)
        self.assertEquals(response.status_code, 200)
        response = self.s3_client.get_bucket_location(Bucket=bucket_name)
        self.assertEqual(response['ResponseMetadata']['HTTPStatusCode'], 200)
        self.assertIn('LocationConstraint', response)
    def test_bucket_policy(self):