sync: implement more traits for channel errors #5666

Update mpsc::error::{SendError<T>, TrySendError<T>, SendTimeoutError<T>} to match the errors from the std lib in their mpsc channel. Removed bond <T: Debug>. Add derived impl PartialEq, Eq, Clone and Copy (only if T implement them)

Update watch::error::SendError<T> to match the error from the std lib in their mpsc channel (also a SendError). Removed bond <T: Debug>. Add derived impl PartialEq, Eq, Clone and Copy (only if T implement them)