Provide a more friendly message to the users

When user used the include the revision which is already included by Fish it will provide an error log to the users:

1. Create new review with the project code "CR".
2. Select in dropdown Add to Review as value Diffs to Last Reviewed Version,
3. Moved back to the revisions page with number 2724623 selected this 2724623 revision, it was successfully added,
4. Then tried to add 2724877 revision and the error appeared.

It will be great if we can improve it to have more friendly message,