Fix: JS: Improve error handling of batch addition

Crash if there is an error in batch addition.
Example:

```
const usearch = require('usearch');
const index = new usearch.Index({ metric: 'l2sq', connectivity: 16, dimensions: 3 });
index.add(42n, new Float32Array([0.2, 0.6, 0.4]));
index.add(
  [41n, 42n, 43n],
  [[0.1, 0.6, 0.4], [0.2, 0.6, 0.4], [0.3, 0.6, 0.4]]
)
```

`42n` is duplicated, which causes an error, but then it crash.

It seems that it is not better to throw an exception during the process,
so we changed it to throw it after it is completed.

javascript/lib.cpp

@@ -165,13 +165,16 @@ void CompiledIndex::Add(Napi::CallbackInfo const& ctx) {
 
     // Create an instance of the executor with the default number of threads
     auto run_parallel = [&](auto vectors) {
+        std::string error = "";
         executor_stl_t executor;
         executor.fixed(tasks, [&](std::size_t /*thread_idx*/, std::size_t task_idx) {
             add_result_t result = native_->add(static_cast<default_key_t>(keys[task_idx]),
                                                vectors + task_idx * native_->dimensions());
             if (!result)
-                Napi::Error::New(ctx.Env(), result.error.release()).ThrowAsJavaScriptException();
+                error += "<key:" + std::to_string(keys[task_idx]) + " message:" + result.error.release() + ">";
         });
+        if (error.size() > 0)
+            Napi::Error::New(ctx.Env(), error).ThrowAsJavaScriptException();
     };
 
     Napi::TypedArray vectors = ctx[1].As<Napi::TypedArray>();