### 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
v8::Handle<v8::Value> val,
                     base::win::ShortcutOperation* out) {
    std::string operation;
    if (!ConvertFromV8(isolate, val, &operation))
      return false;
    if (operation.empty() || operation == "create")
      *out = base::win::SHORTCUT_CREATE_ALWAYS;
    else if (operation == "update")
      *out = base::win::SHORTCUT_UPDATE_EXISTING;
    else if (operation == "replace")
      *out = base::win::SHORTCUT_REPLACE_EXISTING;
    else
      return false;
    return true;
  }
};
}  
#endif
namespace {
void OnOpenExternalFinished(gin_helper::Promise<void> promise,
                            const std::string& error) {
  if (error.empty())
    promise.Resolve();
  else
    promise.RejectWithErrorMessage(error.c_str());
}
v8::Local<v8::Promise> OpenExternal(const GURL& url, gin::Arguments* args) {