`ServiceProcessor.RegisterStreamingHandler` fails to return error to the client when failing to start.

When a streaming handler returns a non-nil error, it is logged but the client is not informed of the call's failure.

```go
s.RegisterStreamingHandler(func(*struct{}) (chan *struct{}, chan bool, error) { return nil, nil, errors.New("") })
```
blocks the client until the channel is closed via timeout.

See #668 for a test case triggering it.