### 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/platform_util_win.cc
return S_OK;
}
HRESULT DeleteFileProgressSink::ResetTimer() {
  return S_OK;
}
HRESULT DeleteFileProgressSink::PauseTimer() {
  return S_OK;
}
HRESULT DeleteFileProgressSink::ResumeTimer() {
  return S_OK;
}
std::string OpenExternalOnWorkerThread(
    const GURL& url,
    const platform_util::OpenExternalOptions& options) {
  base::ScopedBlockingCall scoped_blocking_call(FROM_HERE,
                                                base::BlockingType::MAY_BLOCK);
  
  
  
  base::string16 escaped_url = L"\"" + base::UTF8ToUTF16(url.spec()) + L"\"";
  base::string16 working_dir = options.working_dir.value();
  if (reinterpret_cast<ULONG_PTR>(
          ShellExecuteW(nullptr, L"open", escaped_url.c_str(), nullptr,
                        working_dir.empty() ? nullptr : working_dir.c_str(),
                        SW_SHOWNORMAL)) <= 32) {
    return "Failed to open";
  }
  return "";
}
void ShowItemInFolderOnWorkerThread(const base::FilePath& full_path) {