### 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_linux.cc
std::unique_ptr<base::Environment> env(base::Environment::Create());
  
  std::string trash;
  if (!env->GetVar(ELECTRON_TRASH, &trash)) {
    
    const auto desktop_env(base::nix::GetDesktopEnvironment(env.get()));
    if (desktop_env == base::nix::DESKTOP_ENVIRONMENT_KDE4 ||
        desktop_env == base::nix::DESKTOP_ENVIRONMENT_KDE5) {
      trash = "kioclient5";
    } else if (desktop_env == base::nix::DESKTOP_ENVIRONMENT_KDE3) {
      trash = "kioclient";
    }
  }
  
  std::vector<std::string> argv;
  const auto& filename = full_path.value();
  if (trash == "kioclient5" || trash == "kioclient") {
    argv = {trash, "move", filename, "trash:/"};
  } else if (trash == "trash-cli") {
    argv = {"trash-put", filename};
  } else if (trash == "gvfs-trash") {
    argv = {"gvfs-trash", filename};  
  } else {
    argv = {"gio", "trash", filename};
  }
  return XDGUtil(argv, true);
}
void Beep() {