Fix exceptions never being reported when UniTask is executed without await and Forget()

[SolidAlloy]

Fixes issue #315

I tried following your advice to replace Exception ex with ExceptionHolder. However, it does not resolve the issue because the following happens:

1. ExceptionHolder is created and assigned to the field.
2. Task getter is executed and UniTask.FromException(ex.GetException().SourceException) is returned.
3. GetException() suppresses the ExceptionHolder's finalizer.
4. The returned task is not awaited, and thus the exception is not reported again.

Instead, I found that the issue lies in another part of the code.
AsyncUniTaskMethodBuilder.Task getter is executed even if the task is not assigned to a variable:

private void Awake()
{
    TestAsync();
}

private async UniTask TestAsync()
{
    throw new Exception();
}

Thus, we can't know if the task returned by the Task getter is going to be awaited or not. We need a backup for a case when the task is not going to be awaited.

In the end, I added a finalizer to ExceptionResultSource:

~ExceptionResultSource()
{
    if (!calledGet)
    {
        UniTaskScheduler.PublishUnobservedTaskException(exception.SourceException);
    }
}

It acts the same as ExceptionHolder. If the exception is thrown, the finalizer will be suppressed. But if the task is never awaited, the finalizer will publish the exception.

[timcassell]

You can remove the flag and just call GC.SuppressFinalize.

[SolidAlloy]

Yes, I understand your reasoning. I just copied the code from ExceptionHolder, along with the flag. Maybe neuecc had a reason to put it there .e.g there was a specific case when the finalizer was called regardless of SupressFinalize, so I decided not to remove it. Anyway, it won't hurt the performance

[github-actions]

This PR is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 7 days.

[neuecc]

Ok, thanks!