### Preflight Checklist
* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.

### Issue Details
```js
let ret = shell.openItem('file:///noexistent');
console.log(ret);
```
`ret` is always `true` on Linux even if opening fails, e.g.
```sh
xdg-open 'file:///noexistent'
echo $?
```
Shows nonzero error code.

* **Electron Version:**
  * v6.0.11
* **Operating System:**
  *Linux / Ubuntu 16.04 x64

### Expected Behavior
`ret` should be `false`, if `xdg-open` returns non-zero code.

Optional: It would be very nice if the `xdg-open` error (catch stderr) would be displayed in `dialog.showErrorBox` if it returns non-zero code. Just take care that `xdg-open` may write some warnings to stderr even if it returns 0.

shell/common/platform_util_win.cc
base::ScopedBlockingCall scoped_blocking_call(FROM_HERE,
                                                base::BlockingType::MAY_BLOCK);
  base::win::ScopedCOMInitializer com_initializer;
  if (!com_initializer.Succeeded())
    return;
  base::FilePath dir = full_path.DirName().AsEndingWithSeparator();
  
  if (dir.empty())
    return;
  Microsoft::WRL::ComPtr<IShellFolder> desktop;
  HRESULT hr = SHGetDesktopFolder(desktop.GetAddressOf());
  if (FAILED(hr))
    return;
  base::win::ScopedCoMem<ITEMIDLIST> dir_item;
  hr = desktop->ParseDisplayName(NULL, NULL,
                                 const_cast<wchar_t*>(dir.value().c_str()),
                                 NULL, &dir_item, NULL);
  if (FAILED(hr)) {
    ui::win::OpenFolderViaShell(dir);
    return;
  }
  base::win::ScopedCoMem<ITEMIDLIST> file_item;
  hr = desktop->ParseDisplayName(
      NULL, NULL, const_cast<wchar_t*>(full_path.value().c_str()), NULL,
      &file_item, NULL);