Fix S3 stat 404

Summary:
In the logs, we can see this 404 being translated into a 500:

```
2018-05-23T00:21:51.866Z        ERROR   dockerregistry/blobs.go:79      Failed to download sha256:d317746f2ef22a664bfc4df5303416b8b49443484bf44abe08c24812264c1690: remote backend download: all origins unavailable: origin kraken-origin-master01-dca1:9003: GET http://kraken-origin-master01-dca1:9003/namespace/uber-usi%2Fmilton/blobs/sha256:d317746f2ef22a664bfc4df5303416b8b49443484bf44abe08c24812264c1690 500: stat: NotFound: Not Found
```

It turns out that the Go SDK is broken and HeadObject does not return ErrCodeNoSuchKey
on 404s. See aws/aws-sdk-go#1208

Instead, we must check the string manually.

Reviewers: #kraken, yiran

Reviewed By: #kraken, yiran

Subscribers: jenkins

Differential Revision: https://code.uberinternal.com/D1763887

lib/backend/s3backend/client.go

@@ -177,5 +177,5 @@ func (c *Client) Upload(name string, src io.Reader) error {
 
 func isNotFound(err error) bool {
 	awsErr, ok := err.(awserr.Error)
-	return ok && awsErr.Code() == s3.ErrCodeNoSuchKey
+	return ok && awsErr.Code() == s3.ErrCodeNoSuchKey || awsErr.Code() == "NotFound"
 }