### 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
content::NavigationHandle* navigation_handle) {
  if (navigation_handle->IsInMainFrame() ||
      !navigation_handle->GetURL().SchemeIs("chrome-extension") ||
      !navigation_handle->HasCommitted())
    return;
  content::RenderFrameHost* frame = navigation_handle->GetRenderFrameHost();
  auto origin = navigation_handle->GetURL().GetOrigin().spec();
  auto it = extensions_api_.find(origin);
  if (it == extensions_api_.end())
    return;
  
  
  base::ReplaceFirstSubstringAfterOffset(&it->second, 0, "var chrome",
                                         "var chrome = window.chrome ");
  auto script = base::StringPrintf("%s(\"%s\")", it->second.c_str(),
                                   base::GenerateGUID().c_str());
  
  
  frame->ExecuteJavaScriptForTests(base::UTF8ToUTF16(script),
                                   base::NullCallback());
}
void InspectableWebContentsImpl::SendMessageAck(int request_id,
                                                const base::Value* arg) {
  base::Value id_value(request_id);
  CallClientFunction("DevToolsAPI.embedderMessageAck", &id_value, arg, nullptr);
}
}