Error signal not propagated if writeFunction in ChannelSendOperator fails immediately

[rstoyanchev]

If the writeFunction in ChannelSendOperator fails immediately inside the call to apply, the error signal should be propagated to the write completion Subscriber. Not doing so means the error is lost and the write appears to never complete.

This is unlikely to occur in WebFlux unless one of the Supplier based commit action fails, but is more likely to happen in RSocket processing where the write function invokes a return value handler (and is how it was uncovered).