- Electron version: 1.2.4 (May Be All Version has the issue)
- Operating system: MAC OS X 10.10.5 (May Be All OS)

i set the `DownloadItem.setSavePath` but if the target resource return 404 status from server, then the save dialog prompt.
### Steps to reproduce

Merged in from #6957
1. Disconnect from the network
2. initiate a download using mainWindow.webContents.downloadURL(url);
3. call item.setSavePath(path) in the will-download handler

Result:
The browser will show a save dialog.

Expected result:
No save dialog, just an updated or done callback on the item

It appears that the code that notices a save path has been set and prevents the dialog from coming up doesn't happen when the download fails.

atom/browser/atom_download_manager_delegate.h
#ifndef ATOM_BROWSER_ATOM_DOWNLOAD_MANAGER_DELEGATE_H_
#define ATOM_BROWSER_ATOM_DOWNLOAD_MANAGER_DELEGATE_H_
#include <string>
#include "base/memory/weak_ptr.h"
#include "content/public/browser/download_manager_delegate.h"
namespace content {
class DownloadManager;
}
namespace atom {
class AtomDownloadManagerDelegate : public content::DownloadManagerDelegate {
 public:
  using CreateDownloadPathCallback =