https://paritytech.github.io/polkadot-sdk/master/sp_std/sync/mpmc/struct.SendError.html
SendError in sp_std::sync::mpmc - Rust
An error returned from the `Sender::send` or `SyncSender::send` function on channels.
spstdsyncmpmcrust
https://www.php.net/manual/uk/eventhttprequest.senderror.php
PHP: EventHttpRequest::sendError - Manual
Send an HTML error message to the client
phpmanual