### 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/common/api/atom_api_shell.cc
gin_helper::Promise<void> promise(args->isolate());
  v8::Local<v8::Promise> handle = promise.GetHandle();
  platform_util::OpenExternalOptions options;
  if (args->Length() >= 2) {
    gin::Dictionary obj(nullptr);
    if (args->GetNext(&obj)) {
      obj.Get("activate", &options.activate);
      obj.Get("workingDirectory", &options.working_dir);
    }
  }
  platform_util::OpenExternal(
      url, options,
      base::BindOnce(&OnOpenExternalFinished, std::move(promise)));
  return handle;
}
bool MoveItemToTrash(gin::Arguments* args) {
  base::FilePath full_path;
  args->GetNext(&full_path);
  bool delete_on_fail = false;
  args->GetNext(&delete_on_fail);
  return platform_util::MoveItemToTrash(full_path, delete_on_fail);
}
#if defined(OS_WIN)
bool WriteShortcutLink(const base::FilePath& shortcut_path,