`Prelude.undefined` called from `Network.HTTP2.Client.Run.hs:154:30`

It seems the new exception handling behavior introduced in #137 has uncovered a previously unreachable path. I'm seeing `Prelude.undefined` errors during some of the stress tests for our gRPC library

```
test-stress: Prelude.undefined
CallStack (from HasCallStack):
  undefined, called at ./Network/HTTP2/Client/Run.hs:154:30 in http2-5.3.1-95e9324b:Network.HTTP2.Client.Run
```