- 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
content::DownloadItem* download,
    const content::DownloadTargetCallback& callback) {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  if (!download->GetForcedFilePath().empty()) {
    callback.Run(download->GetForcedFilePath(),
                 content::DownloadItem::TARGET_DISPOSITION_OVERWRITE,
                 content::DOWNLOAD_DANGER_TYPE_NOT_DANGEROUS,
                 download->GetForcedFilePath());
    return true;
  }
  
  {
    v8::Isolate* isolate = v8::Isolate::GetCurrent();
    v8::Locker locker(isolate);
    v8::HandleScope handle_scope(isolate);
    api::DownloadItem* download_item = api::DownloadItem::FromWrappedClass(
        isolate, download);
    if (download_item) {
      base::FilePath save_path = download_item->GetSavePath();
      if (!save_path.empty()) {
        callback.Run(save_path,
                     content::DownloadItem::TARGET_DISPOSITION_OVERWRITE,
                     content::DOWNLOAD_DANGER_TYPE_NOT_DANGEROUS,
                     save_path);
        return true;
      }
    }
  }
  AtomBrowserContext* browser_context = static_cast<AtomBrowserContext*>(
      download_manager_->GetBrowserContext());