### 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
CloseDevTools();
}
content::ColorChooser* InspectableWebContentsImpl::OpenColorChooser(
    content::WebContents* source,
    SkColor color,
    const std::vector<blink::mojom::ColorSuggestionPtr>& suggestions) {
  auto* delegate = web_contents_->GetDelegate();
  if (delegate)
    return delegate->OpenColorChooser(source, color, suggestions);
  return nullptr;
}
void InspectableWebContentsImpl::RunFileChooser(
    content::RenderFrameHost* render_frame_host,
    std::unique_ptr<content::FileSelectListener> listener,
    const blink::mojom::FileChooserParams& params) {
  auto* delegate = web_contents_->GetDelegate();
  if (delegate)
    delegate->RunFileChooser(render_frame_host, std::move(listener), params);
}
void InspectableWebContentsImpl::EnumerateDirectory(
    content::WebContents* source,
    std::unique_ptr<content::FileSelectListener> listener,
    const base::FilePath& path) {
  auto* delegate = web_contents_->GetDelegate();
  if (delegate)
    delegate->EnumerateDirectory(source, std::move(listener), path);
}
void InspectableWebContentsImpl::OnWebContentsFocused(