fix stack traces from errors in ToAsyncEnumerable

[adamjones2]

Resolves #2301

Now the repro code prints:

System.Exception: test
   at Test.Throw(Int32 x) in Program.cs:line 601
   at System.Reactive.Linq.ObservableImpl.Select`2.Selector._.OnNext(TSource value)
--- End of stack trace from previous location ---
   at System.Linq.AsyncEnumerable.ObservableAsyncEnumerable`1.MoveNextCore() in /_/Ix.NET/Source/System.Linq.Async/System/Linq/Operators/ToAsyncEnumerable.Observable.cs:line 98
   at System.Linq.AsyncIteratorBase`1.MoveNextAsync() in /_/Ix.NET/Source/System.Linq.Async/System/Linq/AsyncIterator.cs:line 70
   at System.Linq.AsyncIteratorBase`1.MoveNextAsync() in /_/Ix.NET/Source/System.Linq.Async/System/Linq/AsyncIterator.cs:line 75
   at Program.<Main>$(String[] args) in Program.cs:line 68
   at Program.<Main>$(String[] args) in Program.cs:line 68