The ToObservable operator can throw unhandled exception

Hi, and happy new year! I think that I have found a bug in the [`ToObservable`](https://github.com/dotnet/reactive/blob/main/Ix.NET/Source/System.Linq.Async/System/Linq/Operators/ToObservable.cs) extension method for async-enumerable sequences. In case an exception is thrown by the `GetAsyncEnumerator` invocation, the exception is not propagated by the `OnError` mechanism of the produced observable, and instead it is rethrown as an unhandled exception that crashes the process. Here is a minimal demonstration of this behavior:

```C#
using System;
using System.Linq;
using System.Threading;
using System.Collections.Generic;

public class Program
{
public static void Main()
{
var observable = new BadAsyncEnumerable().ToObservable();
observable.Subscribe(item => { }, error => Console.WriteLine(error.Message));
Thread.Sleep(1000);
}

class BadAsyncEnumerable : IAsyncEnumerable
{
public IAsyncEnumerator GetAsyncEnumerator(
CancellationToken cancellationToken = default)
{
throw new Exception("Oops!");
}
}
}
```

**Expected behavior:** The error message is printed in the console.

**Actual behavior:** Unhandled exception.

[Try it on Fiddle](https://dotnetfiddle.net/TxPl28).