feat(runtime): show the actual error when an async job throws

[wytzepiet]

When a queued job (a promise callback, a setTimeout, etc.) throws, executePendingJob reported just:

Async job raised an exception

The real JavaScript error — its message and stack — was thrown away, so you couldn't tell what actually broke.

Here's why: rquickjs hands back an AsyncJobException whose text is exactly that fixed string. The real error is left on the job's context for the caller to read with Ctx::catch (as its docs note). fjs just wasn't reading it.

This change reads it — the same way the runtime's own idle() already does — and puts it in the error. So now you get the real thing:

Async job raised an exception: Error: Maximum call stack size exceeded
    at render (app.js:42)
    ...

Reading the error also clears it, which fixes a small side bug: the old code left the exception pending, so it could show up again on the next executePendingJob call.

The method signature doesn't change — only the error text gets better.

🤖 Generated with Claude Code

[iota9star]

Review findings:

• The implementation improves the AsyncJobException path, but the PR description/test coverage should be tightened. A normal microtask throw such as Promise.resolve().then(() => { throw new Error(...) }) rejects the promise and does not make execute_pending_job() return Err, so this change does not surface every async callback throw as an immediate execute_pending_job() error.

  I verified this with a temporary test: the first execute_pending_job() returned Ok(true), then the queue drained without this error branch being hit.

  Suggested update: describe the behavior as improving diagnostics when QuickJS reports AsyncJobException from execute_pending_job(), and add a regression test that actually triggers that branch.

• Merge note with fix: prevent JS stack overflow from crashing the process #15: libfjs/src/api/runtime.rs has a manual conflict around execute_pending_job(). The final merged version needs to preserve both behaviors: fix: prevent JS stack overflow from crashing the process #15 stack-executor wrapping and this PR detailed exception extraction via ctx.catch().

Verification run on this PR: cargo test --manifest-path libfjs/Cargo.toml passed locally with 533 tests.