### 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 GURL(base::StringPrintf("%s%s/%s/",
                                 kChromeUIDevToolsRemoteFrontendBase,
                                 kChromeUIDevToolsRemoteFrontendPath,
                                 content::GetWebKitRevision().c_str()));
}
GURL GetDevToolsURL(bool can_dock) {
  auto url_string = base::StringPrintf(kChromeUIDevToolsURL,
                                       GetRemoteBaseURL().spec().c_str(),
                                       can_dock ? "true" : "");
  return GURL(url_string);
}
}  
class InspectableWebContentsImpl::NetworkResourceLoader
    : public network::SimpleURLLoaderStreamConsumer {
 public:
  NetworkResourceLoader(int stream_id,
                        InspectableWebContentsImpl* bindings,
                        std::unique_ptr<network::SimpleURLLoader> loader,
                        network::mojom::URLLoaderFactory* url_loader_factory,
                        const DispatchCallback& callback)
      : stream_id_(stream_id),
        bindings_(bindings),
        loader_(std::move(loader)),
        callback_(callback) {
    loader_->SetOnResponseStartedCallback(base::BindOnce(
        &NetworkResourceLoader::OnResponseStarted, base::Unretained(this)));
    loader_->DownloadAsStream(url_loader_factory, this);
  }
  NetworkResourceLoader(const NetworkResourceLoader&) = delete;