Error when trying to create an existing label in new dropdown

1. Create a new label from the new dropdown
2. Give it an existing name and hit 'Create'
3. The 'Create' button is hung and greyed out
4. You have to refresh the page for the feature to become usable again

cc @iamphill

Ah yeah it should really show an error message.

Reassigned to @iamphill

mentioned in commit 07da8651

mentioned in merge request !3345 (merged)

Milestone changed to 8.6

Added ~12980 label

Added regression label

Status changed to closed by commit 07da8651

mentioned in commit 5b2ce9ad

mentioned in commit 05856199