### 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
const std::string& function_name,
    const base::Value* arg1,
    const base::Value* arg2,
    const base::Value* arg3) {
  if (!GetDevToolsWebContents())
    return;
  std::string javascript = function_name + "(";
  if (arg1) {
    std::string json;
    base::JSONWriter::Write(*arg1, &json);
    javascript.append(json);
    if (arg2) {
      base::JSONWriter::Write(*arg2, &json);
      javascript.append(", ").append(json);
      if (arg3) {
        base::JSONWriter::Write(*arg3, &json);
        javascript.append(", ").append(json);
      }
    }
  }
  javascript.append(");");
  GetDevToolsWebContents()->GetMainFrame()->ExecuteJavaScript(
      base::UTF8ToUTF16(javascript), base::NullCallback());
}
gfx::Rect InspectableWebContentsImpl::GetDevToolsBounds() const {