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.
response_content_str = None
            try:
                response_content_str = to_str(response._content)
            except Exception:
                pass
            
            
            if method == 'GET':
                query_map = urlparse.parse_qs(parsed.query, keep_blank_values=True)
                for param_name, header_name in ALLOWED_HEADER_OVERRIDES.items():
                    if param_name in query_map:
                        response.headers[header_name] = query_map[param_name][0]
            if response_content_str and response_content_str.startswith('<'):
                is_bytes = isinstance(response._content, six.binary_type)
                response._content = response_content_str
                append_last_modified_headers(response=response, content=response_content_str)
                
                
                
                
                
                if method != 'GET' or not is_object_specific_request(path, headers):
                    response._content = re.sub(r'([^\?])>\n\s*<', r'\1><', response_content_str, flags=re.MULTILINE)
                
                response._content = self._update_location(response._content, bucket_name)
                
                if is_bytes:
                    response._content = to_bytes(response._content)