channel: Implement Display and Error for TryReadyError and ReadyTimeoutError - #1327

TryReadyError and ReadyTimeoutError are public, re-exported from crossbeam-channel/src/lib.rs:383, and are what Select::try_ready, Select::ready_timeout and Select::ready_deadline return. They are the only error types in crossbeam-channel/src/err.rs that implement neither fmt::Display nor std::error::Error, so the select side works and the ready side does not:

// fine
let _: Box<dyn std::error::Error> = Box::new(crossbeam_channel::SelectTimeoutError);
// error[E0277]: the trait bound `ReadyTimeoutError: std::error::Error` is not satisfied
let _: Box<dyn std::error::Error> = Box::new(crossbeam_channel::ReadyTimeoutError);

In practice that means sel.ready_timeout(d)? does not compile in a function returning Box<dyn Error> or anyhow::Result, while sel.select_timeout(d)? does, and println!("{err}") on either ready error does not compile at all. Today only Debug is available, from the derive on the two structs.

This adds the two Display impls and the two empty error::Error impls at the end of err.rs, which keeps the impls in the order the types are declared. The messages are the ones already used by the select counterparts at crossbeam-channel/src/err.rs:340-354. TrySelectError and TryReadyError carry the same doc line, "Failed because none of the channel operations were ready.", so both say "all operations in select would block"; SelectTimeoutError and ReadyTimeoutError carry the same doc line, "Failed because none of the channel operations became ready before the timeout.", so both say "timed out waiting on select".

error_impls in crossbeam-channel/tests/ready.rs boxes each error as Box<dyn Error> and asserts the rendered text. It runs on one thread with no sleeps and no timing assumptions, so it is fine under Miri and on every CI target. ready_timeout(ms(0)) reaches the deadline branch of run_ready deterministically, because ready_timeout computes the deadline as Instant::now() before the readiness scan and the scan is what advances the clock past it. The sender stays bound as _s so the channel is connected and never ready.

Verified on macOS aarch64 with cargo 1.95.0:

• Without the err.rs change, cargo test -p crossbeam-channel --test ready error_impls fails to build with two error[E0277], one per type.
• With it, test error_impls ... ok.
• cargo test -p crossbeam-channel: 400 passed, 0 failed, 1 ignored across 15 test binaries, plus 74 doctests passed.
• cargo clippy -p crossbeam-channel --lib --tests -- -D warnings: clean. --all-targets cannot be used on stable here because benches/crossbeam.rs opens with #![feature(test)]; that fails identically on an unmodified checkout.
• cargo fmt --all -- --check: clean.

Nothing public is removed or changed, so the addition is backwards compatible.