remotes: fix dockerPusher to handle abort correctly

`dockerPusher` provides `pushWriter` which implements `content.Writer`.
However, even if `pushWriter` become abort status (i.e. `Close()` is called
before `Commit()`), `dockerPusher` doesn't recognise that status and treats that
writer as on-going.
This behaviour doesn't allow the client to retry an aborted push.

This commit fixes this issue.
This commit also adds an test to ensure that the issue is fixed.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

remotes/docker/status.go

@@ -31,6 +31,9 @@ type Status struct {
 
 	Committed bool
 
+	// ErrClosed contains error encountered on close.
+	ErrClosed error
+
 	// UploadUUID is used by the Docker registry to reference blob uploads
 	UploadUUID string
 }