QuickJS: fix error reporting in query server

[Karthikeya1500]

QuickJS: fix error reporting in query server

Problem

The QuickJS query server fails to report useful error messages when standard JavaScript errors or non-Error objects (like strings) are thrown outside of captured sandboxes.

1. For standard Error objects: JSON.stringify(e) results in an empty object {}. The client receives ["error", "SomeError", {}] instead of the error details.
2. For non-Error objects (e.g., throw "foo"): The server attempts to access e.stack, which is undefined on strings/primitives. This results in ["error", "unnamed_error", null].

Solution

Use the existing errstr(e) utility (already defined in util.js) to ensure that a descriptive string representation of the error is sent back to the client. This brings the QuickJS implementation in line with the SpiderMonkey behavior in loop.js and other parts of the codebase.

Testing

Verified that errstr(e) correctly identifies and stringifies both standard Error instances and primitive values thrown by user code.

[nickva]

Thanks @Karthikeya1500!

errstr(e) for QuickJS will just call e.toString(). For Spidermonkey it handles a special case for an a SM-only function. So we could just define local QuickJS helper here or just call e.toString().

For some errors which are not Error objects, e.toString() or errstr(e) will fail I think? We check above for null as a special case, but I guess undefined also won't have e.toString() so maybe in a helper function we should check for that. Symbols may another thing which won't work well.

Wonder something like this could work:

try {`${e} stack: ${e.stack}`} catch {JSON.stringify(e)};

That is, we try to format both the error (its type and name) and the stack as a string, and if that fails, say, we get an undefined then we return what we did before.

Ideally we'd add test cases like the one we added in the previous pull request. Since we may have some behavior divergence between Spidermonkey and QuickJS we may have to use a different check depending on what engine is being used like we do here: https://github.com/apache/couchdb/blob/main/src/couch/test/eunit/couch_js_tests.erl#L445-L467

Also, please fill out the regular PR template we have for pull requests.