For the link controller, the error handling has been added.
As the redaction system use ajax to update links, the controller return error
result in JSON format.

To handle validation errors correctly on client side, the edition_in_place
client javascript has to handle ajax error event:

when error is raised and response is JSON, event handler looks for error
messages and display them in the "ul.error" HTML tag.

Furthermore, the [`render text` has been deprecated](https://github.com/rails/rails/issues/12374)
I had to replace them with `render plain` for redaction locks.