### 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
NetworkResourceLoader& operator=(const NetworkResourceLoader&) = delete;
 private:
  void OnResponseStarted(const GURL& final_url,
                         const network::mojom::URLResponseHead& response_head) {
    response_headers_ = response_head.headers;
  }
  void OnDataReceived(base::StringPiece chunk,
                      base::OnceClosure resume) override {
    base::Value chunkValue;
    bool encoded = !base::IsStringUTF8(chunk);
    if (encoded) {
      std::string encoded_string;
      base::Base64Encode(chunk, &encoded_string);
      chunkValue = base::Value(std::move(encoded_string));
    } else {
      chunkValue = base::Value(chunk);
    }
    base::Value id(stream_id_);
    base::Value encodedValue(encoded);
    bindings_->CallClientFunction("DevToolsAPI.streamWrite", &id, &chunkValue,
                                  &encodedValue);
    std::move(resume).Run();
  }
  void OnComplete(bool success) override {