Expose the customized API errors to plugins

[Aschen]

Description

Customized errors has been introduced to application as a standard way to return API errors according with the business layer.

Plugins should be able to register their own errors with a predefined domain matching the plugin name.

Other change

It's not possible to throw an internal Kuzzle error from an application (e.g. throw app.errors.get('api', 'assert', 'invalid_type', 'string', 'date') see doc)

Kuzzle predefined errors should also be accessible to plugins and applications