don't swallow errors in Swift driver's GetContent()

In 326c3a9c49, which was only intended to
be a refactoring commit, the behavior of this block subtly changed so
that unknown types of errors would be swallowed instead of propagated.

I noticed this while investigating an error similar to #1539 aka
docker/docker#21290. It appears that during GetContent() for a
hashstate, the Swift proxy produces an error. Since this error was
silently swallowed, an empty []byte is used to restart the hash, then
producing the digest of the empty string instead of the layer's digest.

This PR will not fix the issue, but it should make the actual error more
visible by propagating it into `blobWriter#resumeDigest' and
'blobWriter#validateBlob', respectively.

Signed-off-by: Stefan Majewsky <stefan.majewsky@sap.com>

registry/storage/driver/swift/swift.go

@@ -280,7 +280,7 @@ func (d *driver) GetContent(ctx context.Context, path string) ([]byte, error) {
 	if err == swift.ObjectNotFound {
 		return nil, storagedriver.PathNotFoundError{Path: path}
 	}
-	return content, nil
+	return content, err
 }
 
 // PutContent stores the []byte content at a location designated by "path".