When stop() or stopOnError rejected queued task promises, the rejection
handlers (via Promise.allSettled) weren't attached yet, causing Mozilla
to report each one as "uncaught exception: Object". Add no-op catch
handlers before rejecting so the runtime knows they'll be handled
downstream. Also fix stopOnError path to reject with CanceledException
instead of undefined, and remove legacy Bluebird handledRejection flag.
Previously, we simply cleared the queue, leaving any pending promises
unresolved, and we checked for isResolved()/isPending() in
storageEngine.js (and maybe other places). storageEngine.js was changed
to simply await those promises, but that results in a hang if the queue
is stopped, since the promises were never resolved. Instead, when
stopping the queue, reject queued promises with a CanceledException and
ignore those promises in storageEngine.js.
