### 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/browser/ui/inspectable_web_contents_impl.cc
const int stream_id_;
  InspectableWebContentsImpl* const bindings_;
  std::unique_ptr<network::SimpleURLLoader> loader_;
  DispatchCallback callback_;
  scoped_refptr<net::HttpResponseHeaders> response_headers_;
};
InspectableWebContentsView* CreateInspectableContentsView(
    InspectableWebContentsImpl* inspectable_web_contents_impl);
const InspectableWebContentsImpl::List& InspectableWebContentsImpl::GetAll() {
  return g_web_contents_instances_;
}
void InspectableWebContentsImpl::RegisterPrefs(PrefRegistrySimple* registry) {
  registry->RegisterDictionaryPref(kDevToolsBoundsPref,
                                   RectToDictionary(gfx::Rect(0, 0, 800, 600)));
  registry->RegisterDoublePref(kDevToolsZoomPref, 0.);
  registry->RegisterDictionaryPref(kDevToolsPreferences);
}
InspectableWebContentsImpl::InspectableWebContentsImpl(
    content::WebContents* web_contents,
    PrefService* pref_service,
    bool is_guest)
    : frontend_loaded_(false),
      can_dock_(true),
      delegate_(nullptr),
      pref_service_(pref_service),