fix: return 404 instead of 412 when dataset not found (#1176)

[Shamarvey1]

Closes #1176

Problem
Currently, when requesting a dataset that does not exist, the API returns HTTP 412 (Precondition Failed). However, according to standard HTTP semantics, a missing resource should return 404 (Not Found).

Solution
Updated the dataset retrieval endpoint to return HTTP 404 when the dataset is not found.

Specifically:

• Modified the data($data_id) function in Api_data.php
• When $dataset === false, the response now uses:
  returnError(111, $this->version, 404)

Why this approach

• Keeps existing internal error code (111) unchanged
• Only updates HTTP response code for correctness
• Minimal and safe change without affecting other endpoints

Note
Other endpoints still use HTTP 412 for similar cases. This PR focuses on the main dataset retrieval endpoint. I can extend this behavior across other endpoints if desired.