S3 FileExists fix

According to https://tools.ietf.org/html/rfc7231#section-4.3.2 HEAD
must not have response body so the AWS error code NoSuchKey
cannot be received from S3 and we need to fallback to HTTP NotFound
error code.

s3/public.go

@@ -20,6 +20,8 @@ import (
 	"github.com/smira/go-aws-auth"
 )
 
+const errCodeNotFound = "NotFound"
+
 // PublishedStorage abstract file system with published files (actually hosted on S3)
 type PublishedStorage struct {
 	s3               *s3.S3
@@ -429,7 +431,7 @@ func (storage *PublishedStorage) FileExists(path string) (bool, error) {
 	_, err := storage.s3.HeadObject(params)
 	if err != nil {
 		aerr, ok := err.(awserr.Error)
-		if ok && aerr.Code() == s3.ErrCodeNoSuchKey {
+		if ok && aerr.Code() == errCodeNotFound {
 			return false, nil
 		}