Correctly shows label errors in dropdown

## What does this MR do?

Correctly shows any errors in the dropdown when creating a new label

## What are the relevant issue numbers?

Closes #18344 

## Screenshots (if relevant)

![Screen_Shot_2016-06-09_at_09.01.59](/uploads/58dc56b39db20a50adce6f12510025fc/Screen_Shot_2016-06-09_at_09.01.59.png)

See merge request !4553