message: refactor(h2): change error message when poll capacity is canceled
diff: @@ -131,7 +131,14 @@ where
  Some(Err(e)) => {
  return Poll::Ready(Err(crate::Error::new_body_write(e)))
  }
  - None => return Poll::Ready(Err(crate::Error::new_canceled())),
  + None => {
  + // None means the stream is no longer in a
  + // streaming state, we either finished it
  + // somehow, or the remote reset us.
  + return Poll::Ready(Err(crate::Error::new_body_write(
  + "send stream capacity unexpectedly closed"
  + )));
  + }
  }
  }
  } else if let Poll::Ready(reason) = me
  