### 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::DevToolsAgentHost* agent_host,
    const std::string& message) {
  if (!frontend_loaded_)
    return;
  if (message.length() < kMaxMessageChunkSize) {
    base::string16 javascript =
        base::UTF8ToUTF16("DevToolsAPI.dispatchMessage(" + message + ");");
    GetDevToolsWebContents()->GetMainFrame()->ExecuteJavaScript(
        javascript, base::NullCallback());
    return;
  }
  base::Value total_size(static_cast<int>(message.length()));
  for (size_t pos = 0; pos < message.length(); pos += kMaxMessageChunkSize) {
    base::Value message_value(message.substr(pos, kMaxMessageChunkSize));
    CallClientFunction("DevToolsAPI.dispatchMessageChunk", &message_value,
                       pos ? nullptr : &total_size, nullptr);
  }
}
void InspectableWebContentsImpl::AgentHostClosed(