[web] Improve error messages in Future.toJS

[ditman]

The current implementation of Future.toJS (here, here) handles errors coming from the Dart future by rejecting a JS Promise.

This is great so JS code knows that something failed within Dart, but the debugging experience of what happened is not great, because the error text is hardcoded to:

Dart exception thrown from converted Future. Use the properties 'error' to fetch the boxed error and 'stack' to recover the stack trace.

As a JS programmer, I find it odd that I need to unbox objects contained in the emitted error, instead of having a textual description of what's wrong.

In the Flutter engine we're trying something like this, but it'd be nice to have a Future.toJS that exposes the "text" of the underlying error directly in the JS console, so we can stop using a bespoke solution.

[dart-github-bot]

Summary: The Future.toJS method currently provides a generic error message when a Dart future fails, making it difficult for JavaScript developers to understand the root cause of the error. The user proposes improving the error message by directly exposing the error text in the JS console, enhancing debugging experience.