Ensure error message stringified safely

[dodo920306]

Previously, api.common.response.err() in api-engine was used in two contexts:

1. To pass plain error messages (strings).
2. To pass Exception.args from the API engine, which is a Python tuple.

Since JavaScript’s template strings call .toString() on non‑string values, passing a tuple resulted in [object Object] in the UI.

This change normalizes the error payload so that:

• Strings pass through unchanged.
• Arrays and objects are serialized via JSON.stringify().
• Other types are converted with String().

As a result, all error messages should display correctly in the frontend without [object Object] artifacts.