- 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
base::Callback<void(const base::FilePath&)>;
  explicit AtomDownloadManagerDelegate(content::DownloadManager* manager);
  virtual ~AtomDownloadManagerDelegate();
  
  void CreateDownloadPath(const GURL& url,
                          const std::string& suggested_filename,
                          const std::string& content_disposition,
                          const std::string& mime_type,
                          const base::FilePath& path,
                          const CreateDownloadPathCallback& callback);
  void OnDownloadPathGenerated(uint32_t download_id,
                               const content::DownloadTargetCallback& callback,
                               const base::FilePath& default_path);
  
  void Shutdown() override;
  bool DetermineDownloadTarget(
      content::DownloadItem* download,
      const content::DownloadTargetCallback& callback) override;
  bool ShouldOpenDownload(
      content::DownloadItem* download,
      const content::DownloadOpenDelayedCallback& callback) override;
  void GetNextId(const content::DownloadIdCallback& callback) override;
 private:
  content::DownloadManager* download_manager_;
  base::WeakPtrFactory<AtomDownloadManagerDelegate> weak_ptr_factory_;
  DISALLOW_COPY_AND_ASSIGN(AtomDownloadManagerDelegate);
};
}  
#endif