fix(run): keep an error listener on pooled idle workers (#51)

* fix(run): keep an error listener on pooled idle workers

A worker released back to the idle pool had its invocation-scoped
listeners detached and only an 'exit' handler reattached, so an 'error'
emitted while it sat parked reached a bare EventEmitter and threw as an
uncaught exception in the host process.

Attach the same discard handler to 'error', which marks the worker
destroyed and drops it from the pool, and remove it again on
reacquisition before the invocation-scoped handlers go on.

* chore: add changeset