Fix ToObservable operator can throw unhandled exception

[fedeAlterio]

Fixes #1677
The issue was that exceptions thrown by GetAsyncEnumerator() were not being propagated to observer.OnError(), resulting in unhandled exceptions. This has been addressed by wrapping the GetAsyncEnumerator() call in a try-catch block

The same issue appears also on DisposeAsync(). This is more complex, because we cannot rely on the observer anymore, because it is already completed at that point.
Currently the exceptioncauses the process to crash directly. In my opinion this is bad since there is nothing that warns the caller about that, and also it does not offer any solution.

Observable.FromAsync already "solves" this problem, so I used the same approach here, creating an overload where is possible to specify if suppress the exception or not. If not, the exception would be routed to TaskScheduler.UnobservedTaskException, rather that be rethrown on the ThreadPool / Synchronization Context directly (causing with 99% probability the process to crash)

[fedeAlterio]

@dotnet-policy-service agree