Fix #1200 – JSON data/tag endpoint returns XML errors

[agrim-rai]

Adds JSON error responses for API v1 endpoints. When clients request /api_new/v1/json/*, errors now return JSON instead of XML.

Solves #1200
Before :

After :

Changes

• Added openml_OS/views/pages/api_new/v1/json/error-message.tpl.php to provide consistent JSON-formatted error responses.
• New JSON error schema:

  {
    "error": {
      "code": "<code>",
      "message": "<message>",
      "additional_information": "<info>"
    }
  }