### 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
return point.x() > rect.x() && point.x() < (rect.width() + rect.x()) &&
         point.y() > rect.y() && point.y() < (rect.height() + rect.y());
}
bool IsPointInScreen(const gfx::Point& point) {
  for (const auto& display : display::Screen::GetScreen()->GetAllDisplays()) {
    if (IsPointInRect(point, display.bounds()))
      return true;
  }
  return false;
}
void SetZoomLevelForWebContents(content::WebContents* web_contents,
                                double level) {
  content::HostZoomMap::SetZoomLevel(web_contents, level);
}
double GetNextZoomLevel(double level, bool out) {
  double factor = blink::PageZoomLevelToZoomFactor(level);
  size_t size = base::size(kPresetZoomFactors);
  for (size_t i = 0; i < size; ++i) {
    if (!blink::PageZoomValuesEqual(kPresetZoomFactors[i], factor))
      continue;
    if (out && i > 0)
      return blink::PageZoomFactorToZoomLevel(kPresetZoomFactors[i - 1]);
    if (!out && i != size - 1)
      return blink::PageZoomFactorToZoomLevel(kPresetZoomFactors[i + 1]);
  }
  return level;
}
GURL GetRemoteBaseURL() {