Function end_at_first_error 

fn end_at_first_error<T, S>(items: S) -> BoxStream<'static, Result<T, Status>>
where
    T: Send + 'static,
    S: Stream<Item = Result<T, Status>> + Send + 'static,

A conversion error is sent as an event: error frame. Ending the stream there means a subscriber that skips the frame cannot miss a height without noticing.