- 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.cc
uint32_t download_id,
    const content::DownloadTargetCallback& callback,
    const base::FilePath& default_path) {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  auto item = download_manager_->GetDownload(download_id);
  if (!item)
    return;
  NativeWindow* window = nullptr;
  content::WebContents* web_contents = item->GetWebContents();
  auto relay = web_contents ? NativeWindowRelay::FromWebContents(web_contents)
                            : nullptr;
  if (relay)