Back-end validation for approve election

In the current system user can reject the elections which are approved and used for elections

Steps to reproduce the behavior:
1. Go to Election approval
2. Click on Approved elections tab
3. Select an election which has been used for previous election
4. Click on view button
5. Scroll down to the bottom and click on Reject button
6. Give a comment on dialog and click on Save

**Expected behavior**
Provide informative error messages when the user try to reject those kind of elections.