The panic was caused by the lack of an error class mapping for
futures::channel::TrySendError, but it shouldn't have been throwing an error in
the first place - when a worker has terminated, postMessage should just return.
The issue was that the termination message hadn't yet been recieved, so it was
carrying on with trying to send the message. This adds another check on the Rust
side for if the channel is closed, and if it is the worker is treated as
terminated.
