### 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
web_contents_(web_contents),
      is_guest_(is_guest),
      view_(CreateInspectableContentsView(this)),
      weak_factory_(this) {
  const base::Value* bounds_dict = pref_service_->Get(kDevToolsBoundsPref);
  if (bounds_dict->is_dict()) {
    devtools_bounds_ = DictionaryToRect(bounds_dict);
    
    if (devtools_bounds_.height() < 100 || devtools_bounds_.width() < 100) {
      devtools_bounds_.set_height(600);
      devtools_bounds_.set_width(800);
    }
    if (!IsPointInScreen(devtools_bounds_.origin())) {
      gfx::Rect display;
      if (!is_guest && web_contents->GetNativeView()) {
        display = display::Screen::GetScreen()
                      ->GetDisplayNearestView(web_contents->GetNativeView())
                      .bounds();
      } else {
        display = display::Screen::GetScreen()->GetPrimaryDisplay().bounds();
      }
      devtools_bounds_.set_x(display.x() +
                             (display.width() - devtools_bounds_.width()) / 2);
      devtools_bounds_.set_y(
          display.y() + (display.height() - devtools_bounds_.height()) / 2);
    }
  }
  g_web_contents_instances_.push_back(this);
}
InspectableWebContentsImpl::~InspectableWebContentsImpl() {