fix: use errstr() instead of toSource() in nouveau handleIndexError for QuickJS compatibility

[vamsi2246]

Overview

handleIndexError() in nouveau.js calls err.toSource(), which is a SpiderMonkey-specific method not available in QuickJS. This causes the error handler itself to crash when a Nouveau index function throws a non-fatal error on QuickJS-based deployments.

This replaces err.toSource() with the existing errstr() helper (defined in util.js), which safely checks for toSource availability before calling it, falling back to toString(). This is consistent with the identical pattern used in dreyfus.js and views.js.

Testing recommendations

N/A - This is a trivial 1-line compatibility fix that aligns nouveau.js with the existing pattern in dreyfus.js.

[nickva]

Thanks for your contribution, @vamsi2246

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Coincidentally, there is a parallel discussion in another PR #5943 (review) about how errstr can be improved. It's great to see a lot of activity and interest in improving js dispatching in CouchDB!

Wonder if we could cover this case by a test. It should be enough just to copy and paste a test from nouveau Elixir test and make it throw, like so:

  test "index function throws", context do
    db_name = context[:db_name]
    create_search_docs(db_name)
    ddoc =  %{
      nouveau: %{
        bar: %{
          default_analyzer: "standard",
          index: """
            function (doc) {
              throw(new Error("some error"));
              index("string", "foo", doc.foo, {store: true});
            }
          """
        }
      }
    }
    create_ddoc(db_name, ddoc)
    url = "/#{db_name}/_design/foo/_nouveau/bar"
    resp = Couch.post(url, body: %{q: "foo:bar", include_docs: true})
    assert_status_code(resp, 200)
    assert resp.body["hits"] == []
  end

Currently with QuickJS it should blow up with a 500 response and some ugly looking stacktrace. With this fix it should 1) return a 200 response 2) return no hits, 3) hopefully log something like "OS Process ... function raised ...". I don't think we can check for that in elixir but we hopefully will log locally.