Merge branch 'label-flash' into 'master'

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

CHANGELOG

@@ -146,6 +146,9 @@ v 8.10.3
   - Fix importer for GitHub Pull Requests when a branch was removed. !5573
   - Ignore invalid IPs in X-Forwarded-For when trusted proxies are configured. !5584
   - Trim extra displayed carriage returns in diffs and files with CRLFs. !5588
+  - Fix label already exist error message in the right sidebar.
+
+v 8.10.3 (unreleased)
 
 v 8.10.2
   - User can now search branches by name. !5144

app/assets/javascripts/labels_select.js

@@ -70,13 +70,15 @@
               name: newLabelField.val(),
               color: newColorField.val()
             }, function(label) {
-              var errors;
               $newLabelCreateButton.enable();
               if (label.message != null) {
-                errors = _.map(label.message, function(value, key) {
-                  return key + " " + value[0];
-                });
-                return $newLabelError.html(errors.join("<br/>")).show();
+                var errorText = label.message;
+                if (_.isObject(label.message)) {
+                  errorText = _.map(label.message, function(value, key) {
+                    return key + " " + value[0];
+                  }).join('<br/>');
+                }
+                return $newLabelError.html(errorText).show();
               } else {
                 return $('.dropdown-menu-back', $dropdown.parent()).trigger('click');
               }