Fix error message for existing labels in right sidebar.

## What does this MR do?
Fixes flash notification in right sidebar, if we want to create an existing label.

## Are there points in the code the reviewer needs to double check?
Nope.

## Why was this MR needed?
Because it was broken.

## What are the relevant issue numbers?
Fixes #20117 

## Screenshots (if relevant)

#### Before
![https://gitlab.com/gitlab-org/gitlab-ce/uploads/c1fa7e98d1a3f0daaf8197809bbb26d0/Screen_Shot_2016-07-22_at_11.33.35.png](https://gitlab.com/gitlab-org/gitlab-ce/uploads/c1fa7e98d1a3f0daaf8197809bbb26d0/Screen_Shot_2016-07-22_at_11.33.35.png)

#### After
![Screen_Shot_2016-07-28_at_13.38.03](/uploads/1dca73bf4c1b14c0044952a5032d9325/Screen_Shot_2016-07-28_at_13.38.03.png)
![Screen_Shot_2016-07-28_at_13.41.18](/uploads/9b814f7f03d0314050659e1ac6ace290/Screen_Shot_2016-07-28_at_13.41.18.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- Tests
  - [ ] ~~Added for this feature/bug~~
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5546