Non-empty stack traces for operator exceptions.

[bartdesmet]

Improves #1235 by ensuring there's a StackTrace for operators such as SingleAsync when propagating on OnError exception they've created themselves. Because there's no stack to unwind (observer.OnError goes "the other way"), the exception info is limited to the current frame (e.g. OnCompleted in SingleAsync noticing there was no element and thus propagating an InvalidOperationException). At the very least, it gives a clue about the operator that caused it. Further root cause analysis remains a debugging task (e.g. why the sequence produced no elements) but this can help to steer investigations.