Fix stacktraces when using ObservableDeferred and async/await

[erikjohnston]

So __failure__ is a magic (undocumented) thing that twisted will look for when raising exception during an await: https://github.com/twisted/twisted/blob/twisted-19.10.0/src/twisted/internet/defer.py#L748.

Empirically this seems to Do The Right Thing, but it somewhat feels like we're working against Twisted here. Ideally there would be some sort of ObservableDeferred in Twisted that handled this stuff properly, but there doesn't seem to be.

[richvdh]

For a little more detail: this property is read back when a new Failure is constructed further up the stack: https://github.com/twisted/twisted/blob/twisted-19.10.0/src/twisted/python/failure.py#L302. The net effect is that the stacktrace in that new Failure is that in the original Failure (ie, the one that cased the observed Deferred to fail) rather than the point at which the original Failure was converted into an exception by sending it into an await.

[richvdh]

lgtm