### 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
return XDGUtil({"xdg-open", path}, wait_for_exit);
}
bool XDGEmail(const std::string& email, const bool wait_for_exit) {
  return XDGUtil({"xdg-email", email}, wait_for_exit);
}
}  
namespace platform_util {
void ShowItemInFolder(const base::FilePath& full_path) {
  base::FilePath dir = full_path.DirName();
  if (!base::DirectoryExists(dir))
    return;
  XDGOpen(dir.value(), false);
}
bool OpenItem(const base::FilePath& full_path) {
  return XDGOpen(full_path.value(), false);
}
void OpenExternal(const GURL& url,
                  const OpenExternalOptions& options,
                  OpenExternalCallback callback) {
  
  
  if (url.SchemeIs("mailto"))
    std::move(callback).Run(XDGEmail(url.spec(), false) ? ""
                                                        : "Failed to open");
  else
    std::move(callback).Run(XDGOpen(url.spec(), false) ? "" : "Failed to open");
}
bool MoveItemToTrash(const base::FilePath& full_path, bool delete_on_fail) {