frontend: data-lake: unintuitive duplicate name creation behaviour

### Current behaviour

Attempting to create a variable with the same name as an existing one fails on save, but with no error message, so it just feels like Cockpit is broken.

### Expected or desired behaviour

Failure should be accompanied by an error message (or better yet there should be live validation on the name creation process, with an error message displaying immediately if there is an ID clash).

### Additional context

Discovered [while documenting](https://github.com/bluerobotics/Cockpit-docs/pull/29).

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.

The issue points to the frontend data-lake variable-creation flow but names no file or test. Start by locating the save path and existing duplicate-name or ID validation, then reproduce the silent failure. Done means duplicate names produce a visible error, or immediate validation explains the conflict.

Written by the indexing model from the issue text.