### 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
void InspectableWebContentsImpl::InspectedURLChanged(const std::string& url) {
  if (managed_devtools_web_contents_)
    view_->SetTitle(
        base::UTF8ToUTF16(base::StringPrintf(kTitleFormat, url.c_str())));
}
void InspectableWebContentsImpl::LoadNetworkResource(
    const DispatchCallback& callback,
    const std::string& url,
    const std::string& headers,
    int stream_id) {
  GURL gurl(url);
  if (!gurl.is_valid()) {
    base::DictionaryValue response;
    response.SetInteger("statusCode", 404);
    callback.Run(&response);
    return;
  }
  auto resource_request = std::make_unique<network::ResourceRequest>();
  resource_request->url = gurl;
  resource_request->headers.AddHeadersFromString(headers);
  auto* partition = content::BrowserContext::GetDefaultStoragePartition(
      GetDevToolsWebContents()->GetBrowserContext());
  auto factory = partition->GetURLLoaderFactoryForBrowserProcess();
  auto simple_url_loader = network::SimpleURLLoader::Create(
      std::move(resource_request), MISSING_TRAFFIC_ANNOTATION);
  auto resource_loader = std::make_unique<NetworkResourceLoader>(
      stream_id, this, std::move(simple_url_loader), factory.get(), callback);
  loaders_.insert(std::move(resource_loader));
}
void InspectableWebContentsImpl::SetIsDocked(const DispatchCallback& callback,